Mental Affair

Note that Jean says "In your head", as if there were *anywhere else they could be*.

When two people have psychic sex; from beaming sensual pleasures to each other, to copulating in a dreamscape of your minds' creation.

This rarely leads to physical consequences as it would in the real world, since it happens in the Astral Plane, unless Your Mind Makes It Real. Emotional matters are a different story, since it is otherwise no different from a physical affair.

Some forms of Mind Control and Mental Fusion might eerily feel like this.

Gives Jedi Mind Trick an entirely different ring, doesn't it?

On the less sci-fi side: yes, most people are guilty of the one-sided version of this at some time in their life. Also, some religions consider it to be a sin.

Of course, not to be confused with Mind Screw.

No real life examples, please; at least, not until we have evidence that supernatural abilities exist.

Examples of Mental Affair include:

Anime and Manga

This is the basis behind the Hentai Manga Ne.To.Ge (Lovers in a Cyber World) by Tamaki Nozomu. A couple of teenagers meets in a virtual reality MMORPG realistic enough for them to have sex. They later discover that they are classmates, with an already Belligerent Sexual Tension dynamic. Although this does led to romance (and marriage), unusually for a hentai title the girl insists on "no sex before the wedding"... in the real world, at least.
At the end of Psychic Academy, Mew offers this to Ai when they meet in the Paradream for the third time. He declines. Then he wakes up and learns that she died in the night.

Comics -- Books

X-Men
- One of the most (in)famous examples happened when Cyclops has an affair with Emma Frost. Deserves a special mention because they get walked in on by his wife (another telepath).
- Scott's brother had this too, when he starts dating the school's nurse while in a coma, thanks to the latter's telepathic son. The less we talk about this, the better.
- Continuing the theme of mental affairs, soon after discovering the one between Emma Frost and her husband, Jean proceeded to Mind Rape Emma. One of several on Phoenix's record.
Mind**** and Sistah Spooky in Empowered.

Fan Works

In Sensate Focus, a good Star Trek Slash Fic, Spock states that using their telepathic bond "is an intimacy on par with human marital sexual relations." Later on in the fic, this happens.

Spock let the link swing open between them. Jim felt himself falling, falling into some endless ocean. Then Spock was there, around him, through him, in him. He reached out -- no, he reached within himself -- and embraced the essence that was there and the two were one, then two again.

Very common in Pokémon fanfics involving Gardevior.
The Tin Man fanfic Stand My Ground has "sex in the mindscape" variety DG is recovering from an assassination attempt, and creates a dreamscape to recover in. She manages to pull Cain inside to let him know she's okay.
Quoted from the Pokegirls pokedex entry for Bakuchan (a ghost-type):

Bakuchan are often employed as starters for Tamers with physical disabilities or debilitating Blood Curses that would make it difficult for them to Tame Pokégirls otherwise. The reason for this is simple: Bakuchan can get Tamed in their master's dreams, where such restrictions would not affect a Taming session. Fans of the Bakuchan praise her ability to mold their dreams into their greatest fantasies for their satisfaction as well as hers, down to taking the appearance of a more-beloved harem-sister if necessary.

This is very popular in Doctor Who fanfiction, especially Doctor/Master slash.
Occasionally happens in Star FOX fanfics involving Krystal. Sometimes goes into the weird territory, as mentioned with this scene from the defunct Generations trilogy:

Fox: Your mother's done some fucked-up things to me while I was sleeping.
Marcus: Is she really that sadistic?
Fox: No, it's not mean really, just a way of allowing her to get what she wants.
Marcus: Huh?
Fox: Sex, c'mon, you're eighteen, I thought you would have figured this out by now.
Marcus: That's just what I wanted to hear, now I've got the image in my head...

Films -- Live-Action

Spartan (Sylvester Stallone) and Huxley (Sandra Bullock) in Demolition Man, until Spartan realizes that's all there is to it and immediately takes off his headgear.
This is the only way that vampires (or one species of them, anyway) can have sex in a movie that aired on Syfy. It's not clear whether both parties necessarily experience the event at once, and the human partner in each case was able to envision a partner other than the vampire.
In Dreamscape, a psychic who can enter other people's dreams consummates his relationship with a scientist inside one of her dreams. When she wakes she's furious because he invaded her mental privacy.
The "Earth Way" of having sex in Barbarella.
A variant occurs in X Men First Class. Emma Frost uses her powers to make a Russian General think he's having sex with her while she sits on the couch and watches him make love to empty air. Xavier thought it was a nice trick.

Literature

Happens a few times in The Wind Up Bird Chronicle by Haruki Murakami. The character in question is referred to as a "prostitute of the mind."
A brief episode near the climax (no pun intended) of Otherland has Renie and !Xabbu meeting in a virtual world as pure avatars, without bodies. Their mental union is compared to sex in terms of its intensity and significance. Later, they do it in virtual bodies, and then again "for real".
From Fortress of the Pearl, part of The Elric Saga. While exploring the dream worlds for the Pearl at the Heart of the World, Elric of Melnibone and Oone (the beautiful psychic seer whom helped him enter this realm) have an affair. It's another case where Your Mind Makes It Real, since at the end of the story Oone announces she's pregnant (with twins -- yes, she is a seer); she keeps this a secret from Elric, though.
In The End of Mr. Y the protagonist ends up having sex with someone in an imaginary place called the troposphere (complicated, but it's kind of a part of her mind), to which they have both traveled in spirit.
In Mercedes Lackey's By the Sword, after Kerowyn meets, falls in love with, and is forced to leave Herald Eldan, he continues their love affair in their dreams. Kero is under the impression that they're just vivid dreams she can't shake, and is rather shocked when they finally meet in person again years later, and he knows things she never told him (while awake). They get past it, though.

Live-Action TV

In Star Trek: Enterprise, T'Pol experienced a Mind Rape that started out as a consensual Mental Affair (the other guy refused to get out of her mind when she'd had enough). She ends up getting a fictional analogue to AIDS as a result. Trip also participated in a mating ritual that involved reading his partner's mind and her reading his, which caused a male pregnancy.

Video Games

In Knights of the Old Republic II, Visas (with high influence) will offer this at the Point of No Return.
This is how asari "join" in Mass Effect. Physical contact may or may not be involved. Supposedly, they can choose whether or not to get pregnant from this, and the child will always be another asari, no matter who or what they were with.

Web Comics

In Gene Catlow, the "Sight of the Soul" is like a global, psychic internet... and, as two of the main characters learn the hard way, fooling around there can actually lead to pregnancy—even if their bodies aren't touching, their life-forces are, and that's enough. In fact, it is apparently MORE likely, because the end result is one character getting pregnant who was, otherwise, sterile.