To a certain degree, I think it is also associated with the "Women like thinking about gay sex while Men like thinking about lesbian sex." It's just something not everyone expects to see.
What follows is only a rapid theory. It surely will be dumbly written, part because my english is really rusted, part because I try not to use "technical terminology".
"playing" - or creating - a sim means you can "incarnate" in it.
You "project yourself" in it, you animate it by "putting" inside a part of your attention/emotions, etc.
We could represent like the "possession" of the sim by the player (like in the exorcist:) )
The more the "animated" object seems "familiar" or "desirable", the easier it is to animate it.
Mainly, the process of "projecting yourself in some object" is easier when you can "identify" yourself with it.
It could partly explain the "self sim" effect: it is easier to invest yourself in your sim when you can recognize yourself in it.
But "identification" does not mean "perfect similitude to you". You can easily "identify" to many different thing: a desired object, an object of fear, are easy to identify with.
"cross sexual identification" - the identification to a "person" of different gender - is not easy. gender difference is, for the human mind, something very mysterious - mainly because the body ( anatomically, but also on chemical level) and the personal history make it so that you can NEVER exactly know what it FEELS to be of another gender.
So, you have to "change" the sim to make it more easy to identify with.
A male with feminine traits will be easier to create/play for a woman, because he is "partly" woman: you can use the "woman" part as a firm ground, and concentrate on the remaining "male" parts.
"Acting" completely like a man would be "impossible", so the player has to only play "parts" of the male role.
(all of this is unconscious, or pre-conscious, it's an instinctive, uncontrolled behavior)
Men do exactly the same thing: it is impossible for us to "instinctively" act as woman (or at least we think it is), so our "women" sims often have many manly traits, in personality at least.
Or traits that men believe to be feminine.
The yaoi/gay love works on the same ground.
Pornography works if you identify with one of the "actors" or with the eye looking the scene.
As the other gender's body/sensation (or, in a more generic way, the body, feeling, sensations, of anyone but ourselves) are mysteries, a good part of pornography is about trying to identify to the other gender, and participate in it's pleasure.
For men, it leads to lesbian fascination, but also to the "two men for a girl" thing,etc.
Mainly, in pornography as well as in sim creation/play, we have three opposed and complementary moves:
* try to understand what is different by focusing on what is similar (a man with feminine traits is partly understood, representation of the other gender experiencing known sexual sensations makes identification easier, etc)
* try to burninate the difference, reducing the "other" to something that is not different from you, or of what you think it is
* focus on the mystery
I know I am not clear - and anyway I am not sure it's podium material.
Anyway: everything we create (and playing is creating) comes from within us. We must use a "bit" of ourselves to sculpt our creations. So when we try to create something different from us, we have to use what we "dream"/"think we know"/"can understand" of what we are trying to create. Or at least some tricks making our creation "not so different".