With respect, one reason why 'straight' guys (which, let's face it, usually means closeted gay guys in this context) are 'easier' is because they're less likely to be picky! Sometimes you're the only gay guy they know: they're not likely to be doing the scene or making gay contacts in other ways. I know a few gay guys that cruise 'straight' scenes: science fiction conventions, for example, knowing there'll be bi-curious guys there who will be eminently seduceable and wonderfully desperate for someone to make a move on them.
I've never cruised straight guys in my life, nor have I ever set out to seduce straight guys, but I do have a lot of straight male friends. Most of them would also have a lot of gay male friends, they have eyes in their heads and they can tell if a guy is good loking or not, and those straight guys who I have shagged over the years would be as concerned as I am that the men they want to experiment with are hot and at least in their league so to speak.
I've never been to, nor have any intention of going to a "science fiction convention", and have no idea why it would be a place to look for bi-curious or sexually frustrated men. I'm also interested in why you think these conventions are in some way a straight scene as you put it.
Perhaps it's where I live but the getto mentality which imagines that straight and gay live in segregated worlds where sexual interaction between them is an impossibility or that casual sexual experimentation signifies a latent sexuality which overides the persons self identified sexuality is all alien to me.
I've had sex with women in the past (once or twice), and enjoyed it, I suspect it's possible I might do again sometime, but I'm not Bisexual or Bi-curious, I'm Homosexual.
If a straight guys enjoys the odd bit of cock now and then, perhaps even just the once or twice in his entire life that doesn't automatically make him gay, or bisexual. He could just be experimenting and having a good time nothing more nothing less.