I think pairings are driven by availability and budget: Barry availability is probably 24/7 and I'll bet he comes pretty cheap.
In an interview with Johnny McGovern (DList), Blu Kennedy, aka Corbin Fisher's 'AJ' maintained that
gay models get paid at about 1/3 the rate of straight-identified models (wonder what impact Lucas' recent acknowledgment of his bisexuality is going to have on his paycheck?) While I think it's important to 'consider the source' of this disclosure, I nonetheless I think it's in these guys' interest to hold their cards pretty close to their chest -- don't look like you're enjoying it too much haha! And of course the distinct possibility exists that the character 'Lucas' is bisexual and the real guy is still straight and laughing all the way to the bank
I think the guys start off saying they're straight. However, once they've established a fan base, like Trey, Mitch or Harley, or on that other site, Lucas, Travis, etc. they can be a little more candid about their proclivities.
Now if I were a commodity like Curtis, I'd have an agent working for me -- the Scott Boraz of porn

Seriously, the kid seems to have about as much going on upstairs as your average MLB player. Pretty face and big dick: perhaps he was told to consider a career in gay porn by his high school guidance counselor after his SATs: "now Curtis, you get an automatic 200 points just for filling out the your name . . . "