Offensive and generalizing statement.
I do respect straight guys and I have a lot of straight friends. Actually they are way better friends than gay guys to me. My favourite persons on the planet are actually straight guys.
They asked me in the past, when I came out, things like "how do you know?" and stuff like that. I don't think it's offensive to ask such things. It's all about being confident in what you are. It's just a question, damn. I really don't see the problem if a gay guy try it with a straight man. Just say no. If you are a grown up person, I don't see the problem, honestly. Besides, sexuality is a very fluid concept, so I don't label person based on their sexuality. So if I really like a guy I don't see the problem in trying. Love is love. Just be polite and do not push it when refused.
You're maybe a little bit insecure
