Man on man less intimidating?

To start off I do not watch porn. I'm straight. But somehow like.. pairings of fictional male characters together is slightly less intimidating than a heterosexual pairing... like in fan fiction or something? Nothing graphic just the pairing of them together. For some reason I just find it intimidating when it's a man and a woman and they're both described as really hot and the woman is amazing in bed or something. Why is this?