If they’re neutral about it, or see it as a shared interest, that’s fine. Anything else would make me uncomfortable and I probably wouldn’t want to spend time around them. “Hey, do you think So-and-so is hot?” is OK. “Man, it would be so hot if you banged So-and-so,” is not.
I also get offended by the idea that a woman having sex with another woman wouldn’t count as cheating, for pretty much what Rala said–
People also tend to assume that LGBT rights should be less important to me because I ended up in a relationship with a man, and so I’m generally assumed to be straight. It doesn’t matter! This is what I am, this is how I feel, and if things had gone somewhat differently, I might have ended up equally happy in a relationship with a woman. The fact that I’m in a relationship with a man doesn’t make the same-sex interest any less legit or relevant.