Attract is a bit of a tricky subject, anonymous. So to answer the simpler question first, absolutely, pokémon can be homosexual. In fact, while it’s not the predominant form of sexuality, it’s also not uncommon for pokémon to be attracted to pokémon of the same sex. Likewise, practically any form of sexuality exhibited by humans can be found among pokémon, and some pokémon species also engage in intercourse not only for breeding but also for the sheer pleasure of it. Sex is, in other words, not just for procreation, even when it comes to pokémon.
However, Attract doesn’t really take into consideration sexuality. Not really, anyway. It’s simply a move that forces the victim to let its guard down by perceiving the user as being attractive, not in a sexual or romantic way but instead in an aesthetic way. Pokémon simply don’t want to hit something that looks too pretty to hit, even if that pretty thing is something they wouldn’t normally be attracted to.
As for why it can only be used against Pokémon that don’t match the user’s sex (unless the user has no gender, at which point it fails, regardless of what the target is), scientists aren’t entirely sure. The prevailing theory is that it’s simply a type of empathic wave that’s slightly different when emitted by one sex than it is when emitted by the other and that this slight difference is enough to make it ineffective against members of the same sex.
The Pokédex According to Bill 