Yaoi isn't just about sex. There is sex involved, yes, but that's not all there is. In
fanfiction, there are different classifications of fics.
Yaoi fanfiction can contain
shonen-ai and, in several cases, does.
Yaoi fanfiction can be plotless, read:
smut, or it can have a beautiful storyline that shows the paired characters building a
relationship over a course of time and experience.
Not all the pairings involved are completely unfounded. Take, for example, Gundam Wing and the relationship between Trowa Barton and Quatre Winner. During the series, they develop a close friendship based on similar simple pleasures (such as the love of music) and by getting to know each other. Of course, since the series was intended for young boys, there would not be anything that pointed to an outright homosexual relationship. It is situations like this that fuel the writers into doing what they do and, if you pay close attention, such things happen often enough in just about any anime to make it plausible.
By the same token, there are Japanese anime series that are centered around such relationships. Gravitation and Ai no Kusabi are examples of this. The former deals with the more romantic drawing together of the characters, while the later is a turbulent sexual relationship centered around the master/slave theme.
Typically, in yaoi anime and fanfiction, one can tell the difference between seme (top) and uke (bottom). The seme tends to have narrow shaped eyes while the uke's eyes are large and expressive, like an anime female or child. This isn't, however, exclusive to yaoi anime. Going back to my earlier example with Gundam Wing, Quatre Winner has "uke eyes" while Trowa Barton has "seme eyes". This is also something that one can find in Japanese based video games. Sephiroth of Final Fantasy VII fame has very much a "seme" appearance. In commonplace, non-yaoi syndication, the "seme eyes" tend to denote a villian or strong lead male character.
As for feminizing male characters, that is actually something that is not well received by a large portion of the yaoi fanfiction crowd. This sort of things tends to lead to extreme out-of-character (OOC) behavior which compounds on top of the already OOC inclinations towards homosexuality. To make such characters more feminine takes away an important part of their personality, which is one of the factors that attracts the writers to the characters. Most writers make the mistake of doing so at least once in their works and they are often called on it by the readers. To quote Spider Robinson's Lady Sally - "I like my bullshit plausible."
By my own experience, both as a writer of yaoi fanfiction and as a reader of it, I've found that the vast majority of fellow readers/writers are not young girls. Nearly half of them are over the ages of 25. By the same token, not all of the readers are female either. Nearly forty percent seem to be gay, lesbian, or bisexual.