I'm not inexperienced, I just think that everyone, of both genders, should be treated equally.
Ok, but Guy A doesn't know that, does he? Guy A is doing the right thing by what he knows, which is all he can do. If he wants, however, he can drop a line or two and see if anything bites. If it does, he could move in if he wants to. But at the start of the situation, as far as he's concerned, she's taken, and he can't have her, so he does the right thing. Unless you're saying that not moving in on someone else's partner is a bad thing, and you should do it at all opportunities?