Single gay men, by and large, have absolutely no respect for others' relationships. This goes triple or more for your typical camp queens. They will shamelessly hit on anyone they want, even if that person is in a relationship, and try and fuck them with not a flicker of guilt.
Cheating is absolutely rife in the gay world. Epidemic proportions.
As far as I'm aware, the lesbian relationship stereotype is the opposite; they stop having sex entirely a few months in.