Magnetic Girlfriend

""Oh dear, here we go again, thought Vimes. Why did I have to wait until I was married to become strangely attractive to powerful women? Why not when I was sixteen? I could have done with it then.""

- Night Watch

A common observation in fiction: Once you get a girlfriend, other girls just come naturally.

The lead male character may be nice, kind, and fairly attractive (though probably not a Bishonen). He may adopt stray kittens, help old ladies across the street, do charity work in Third World countries. It doesn't matter, because he's never, ever going to be able to find a girlfriend.

...at least, not until some dazzlingly gorgeous girl swoops down out of nowhere and attaches herself limpet-like to his arm.

Now suddenly, our hero has an embarrassment of riches. Girls who wouldn't even look at him before are now beating down his door. Old acquaintances he can't even remember will appear and claim to have loved him for years. He can walk down the street and have new characters pop up claiming love at first sight.

This can also happen even when the girl is still only a love interest, which makes it even harder for the hero to beat off the other girls and convince her that she's his One Twoo Wuv. (Which she is.) It also makes it harder for him to beat off in general, but that's another story entirely.

As a part of the wish-fulfillment element that defines the Harem Anime genre, the Magnetic Girlfriend will not be jealous of the other girls that enter the equation. This is because she is either too naive to notice their affections, that she is so nice a person she would rather be friends than rivals with them, or that she trusts her boyfriend to not cheat and just doesn't feel threatened by the extra girls. Oddly enough, her upbeat and kind personality is usually enough to win over their friendship, but the race for the guy is most definitely still on.

Note that if the Magnetic Girlfriend is somehow detached, her effect will often wear off. If girls two through whatever even merited flirting with before taking this into account, they certainly don't after.

Not to be confused with Clingy Jealous Girl. Likely springs from the same twisted psychological causes that make Operation: Jealousy sometimes successful (except when failure would get more laughs).

There is some Truth in Television in this. Sometimes, the experience of getting a girlfriend gives men newfound confidence that makes them more attractive to other women, sometimes, the fact that a man isn't on the hunt anymore makes them act more relaxed and natural and thus less awkward, and presumably the fact the man now has a girlfriend signals to other women that there is something desirable about him. (This may or may not bode well for the relationship.)

Very common in Harem Anime.

Anime and Manga

 * Keitaro and Naru in Love Hina.
 * Belldandy and Keichiii Ah! My Goddess, respectively. Keichi's pseudo-rival is only vaguely interested in Bell because she isn't. Sayoko is arguably less interested in Keiichi than annoyed at Belldandy stealing away most of the attention from her classmates, as well as the protective nature of Bell and Kei's wish causing problems for anyone intruding on them.
 * Ai Yori Aoshi.
 * Nagasarete Airantou almost fits this trope to a T. The magnetism, however, is due not to the guy getting a girl, but to the fact that the guy is the guy -the only human male on the island. Hilarity Ensues.
 * Midori no Hibi takes this to extremes - Midori is not only both figuratively and literally attached to Seiji, but her appearance on his right arm sparks a run of luck where quite a few women (and one slightly nerdy boy) fall for Seiji.
 * This is a bit more justified. Seiji was an unlikable Jerkass before Midori started forcing him to change his ways. If it wasn't for Midori, Ayase would have been gang raped, Lucy would have lost her purse and be stuck in Japan, etc.
 * A mild version happens in Seinen manga and Anime Yume De Aetara (aka If I See You In My Dreams). The wimpy Masuo Fuguno falls in love with the beautiful Nagisa, and suddenly a lot of women began to insinuate/try to seduce/trip on him scantly dressed/fall in love with him. Masuo, being a man with no romance experience, doesn't know how to cope with this or convince Nagisa that everything is an misunderstanding; and Nagisa, being almost as inexperienced as him and a bit of Tsundere, doesn't believe him and gives the cold shoulder. Notorious because, even with this setup, doesn't become an harem series.
 * A running gag in the early Video Girl Ai manga and anime is that Youta Moteuchi's name, if you mispronounce the kanji as "Motenai Yo Da," means "Dateless." He did everything in his power to be noticeable, even buying magazines for tips. Once he met  Ai, however, everyone from his Unlucky Childhood Friend, to the Girl Next Door, to a former classmate who had a crush on him   falls head over heels for him.
 * Lala, Rito's Magical Alien Fiance (of sorts), from the To Love Ru manga. Before he met her, he was pretty much just some average guy who can't admit his crush on an old friend (who apparently also has a crush on him that he isn't aware of). After meeting Lala, however, a series of events is set off that sees him unwittingly gathering an Unwanted Harem.
 * Tenchi Muyo!. Poor naive Tenchi frees Ryoko from her prison, where she has been locked for 700 years. Within a matter of days five or six more show up, almost all of whom are capable of blowing up his house at minimum, the planet at maximum.
 * Himari of Omamori Himari. All the Cute Monster Girls (and one Dark Magical Girl) seem to have come out of the woods right after Himari entered Yuuto's life, although that may have more to do with the awakening of his demon-hunting powers when he turned 16.
 * Actually Rinko was there before Himari.
 * But she's just an ordinary girl, not a Cute Monster Girl or Dark Magical Girl, so the point still stands.
 * Negima has an...interesting take on this; when Negi asks to speak to Ku Fei after class, a bunch of the other girls automatically assume he likes her and begin making their own attempts to win him. Of course, after they all work themselves up over it, it turns out that he wanted learn martial arts from her.
 * It also happens in a more traditional sense. Nodoka confesses to him, and explains to the other girls that she likes his maturity and determination. Cue numerous other girls beginning to be attracted to him for the same reasons. Up to and including a robot.
 * Come to think of it, Touma in To Aru Majutsu no Index is like this too. After Index started living with him his encounters with the magic kind quadrupled in a couple of days, and a harem of women from different religions started gathering around him. The clincher here is that Touma lost his memory, so he doesn't even know why Index is living with him.
 * This is actually a subversion, it's stated that his TEACHER and all of his classmates, and a great many other females, are attracted to him, or at least find him cute because of his bad luck. So... he's ironically popular. Just REALLY unlucky.
 * Discussed in Spice and Wolf. A barmaid tells Lawrence that having a beautiful "companion" (who even went to the effort of marking him with her scent) makes him more desirable.

Live Action TV

 * This happens to JD in Scrubs. In the course of an episode he goes from dateless, to having a girlfriend and back, while a random woman in a bar follows this to a tee.
 * Also happens to Mike in Off Centre. It's actually lampshaded and later subverted when he then tries to cheat on his girlfriend. The girls that where all over him suddenly hate his guts.
 * This happens to Dean in the season 6 premiere of Supernatural: he's wearing a wedding ring (although he's not married, he is shacking up with someone at the time) and a waitress at a bar gives him her phone number along with the check. Dean lampshades it to his buddy that he's in the bar with, but it turns out it's really a demon trying to get him alone so she can kill him.

Webcomics

 * In the webcomic YU+ME: dream, once everyone knows Fiona is a lesbian,.
 * Kevin makes this observation in Wapsi Square here.
 * Nancy in Ow, My Sanity, for unusual (but perfectly logical) reasons.
 * Tai of Questionable Content has an interesting theory on the phenomenom.

Western Animation

 * An episode of Batman Beyond has a nerd who (successfully) attempts to emulate this effect by purchasing an illegal android girlfriend. He's instantly considered more popular, and girls who already have boyfriends offer to "double date" with him. Needless to say, things go bad by the end.
 * That is an understatement, given her obsessive and jealous personality and the Unstoppable Rage she flew in when he suggested they "see other people." Not even Batman could save the house.

Theater
""Everyone tells you that the minute you get married, every other woman in the in the world suddenly finds you attractive; well, that's not true. It only affects the kind of women you always wanted to sleep with, but who wouldn't give you the time of day before, and now they're banging down your door and falling to their knees, or at least that's what it feels like, because you can not touch them...""
 * The song "Temptation" from The Last Five Years: