No-Effort Nookie

Generally, entering a sexual relationship with someone involves a level of trust, willing consent, and in non-consensual situations (which most people call rape) domination by force, all of which require some degree of effort on the party wanting the sexual congress to begin.

And then there is this trope.

This is when the sexual encounter happens with no effort on the part of the person wanting the sex, or so little effort it doesn't really count as making one.

In Real Life, this can happen when one or more parties are drunk or high, sex usually being the result of that person(s) wanting the sex being aroused and the usual reservations on needing to know and trust the other party go out the window due to diminished judgment. Alternately, prostitution requires money (or a substitute) on the part of the party wanting sex, but otherwise no other actual effort is needed, since sex is the service being sold in exchange for another good or service.

This trope is why porn with no plot whatsoever and nukige (hentai games with very little plot and lots of sex) exist.

Character Tropes this commonly applies to:

Memetic Sex God: Regardless how much sex they actually had in their source canon, these characters are considered to have this trope apply to them by their fans.
Kavorka Man: Despite being repulsive in some way, these characters still have some form of innate charm, power, or charisma that makes them prone to this trope.
Sex God: This is basically the whole point of their existence, or something they can make happen simply by wishing it to happen.
The Pornomancer

Examples of No-Effort Nookie include:

Advertising

Strongly implied to happen after the cameras stop rolling in any ad where people throw themselves at the person who uses the product being advertised.

Anime and Manga

In Neon Genesis Evangelion, Gendo Ikari manages to have Naoko and Ritsuko Akagi sleep with him, having to exert no effort at all besides letting them know he's interested, and this guarantees him sexual relationships with both women, who wanted him since day one. He wanted the sex for his own reasons, but he didn't have to do much on his end to get it to happen.

Literature

The concept was made famous in the 1970s as "the zipless fuck" by Erica Jong's novel Fear of Flying.

Oral Tradition

Succubi and incubi, according to most lore on the subject, have sex with sleeping men and women via their dreams, often acting on hidden lust, so the sex is almost always desired by the party in question.

Video Games

In Saints Row IV, getting Kinzie to sleep with the Boss is as easy as walking up to her and asking:

Hey Kinzie, wanna fuck?

In the X-Change games, you are railroaded into having sex (or getting raped) most of the time, all you can do is choose who and what route this will happen. However, the third game does give you a chance to make this trope completely voluntary, though this means someone else will suffer should you avert this trope.
Crescendo gives you the option to invoke this trope on a few routes, though the good ends can either avert this trope or lead to sexual encounters that don't have later negative repercussions.