Yeah, I tend to agree with aloofman. Male sex stars can be average in size, looks, and technique, but they must be able to get hard on demand and keep it hard without interruption for as long as it takes to get the shot, often under pretty unromantic conditions. Yes, they're working with good-looking women (better looking now, at least, than they used to be) and they've got fluffers and viagra to keep them hard, but cumming on demand for poor pay (poorer than for the girls, which accounts for the sheer volume of releases, both cinematic and spermatic, from people like Peter North) is neither easy nor appealing once the novelty wears off.
Now this, of course, doesn't explain why male-male stars would be better looking or better hung. They, I imagine, would need to meet the same requirements as straight men, though maybe their schedules are less demanding and two male performers, in the absence of a starlet with silicone breast payments to make, might get paid more, or at least equally. But a few other factors might apply: the willingness to do gay porn might be slightly higher simply because it's still a niche market, putting out titles that uncles and family members are less likely to see or even know about. The unspoken fact about straight porn is that your family members are indeed watching it. That feeling wouldn't be as great with gay porn, since at least your parents are, in all probability, heterosexual.
Anyway, I think Zora's earlier point about the unstudly stars making the average man feel better about himself is a good one. Especially as regards size. You've got to figure that stars that are downright smaller than average--like the pierced and hairless Devin Wolf or the leathery, Hasselhoff-esque Randy West, both of whom are considered good looking only to the very young or very old--are put in to make the average guy feel like he could do porn. It's no surprise, then, that both of those guys appear in more mainstream films than Lexington Steele. Ron Jeremy is an anomaly nobody can explain, although on the Surreal Life this past Winter they kept saying he was 9.5 inches, which even though you can't tell from beneath his massive, swaying gut, is on the bigger side of the porn spectrum. I guess the conclusion is that porn guys, already a self-select group of super-potent males who "know how to use it" and do use it obsessively and without passion, can't be both hung and good looking. Or, at least, they can't be both and get much work. It would upset the average man, perhaps, to see such a performer who might arouse your woman for more than one reason.
Then there are guys like Julian and Vince Voyeur who don't fit this mold. But could it be that they're the result of women finally having more say than they did in the 80s and early 90s? I hope so, because I'm sick of Joey Silvera. He looks like that commedian Gallagher, also famous in the 80s.