Just because he married a girl and had kids with her, that doesn't mean he can't be a homosexual. People still do that today.

Also, Sam and Frodo weren't best friends before the events of the books/movies. At least I;m pretty sure of that....

I am going to just leave it at I don't care about Sam either way, he could have a gay orgy with some other hobbits and I would just say, "that's gross."