I don't think it's gays who are offended by that generalization... it's the males who claim to be heterosexual that are. I mean, it's a fair generalization when backed up by your points in the OP.
But at the same time, if someone tells you they're straight... you need to let it go and leave them alone.
Also, I think the general idea in the OP rarely applies to males outside of the U.S. People who are not from the U.S. have totally different perspectives on this stuff. A transfer student at my college was a total Madonna STAN and huge Britney fan but he was for sure 100% straight. It's just different overseas...