Maria Pawlowska has a theory as to why the vast majority of straight men don't want to be anally penetrated despite "good physiological reasons" that it can cause "mind-blowing orgasms":
[B]elieve it or not, but according to research in this area most heterosexual men exclude the possibility of using dildos precisely because they don’t want to act "gay" and be "passive." It’s got nothing to do with what they like or dislike in bed (mostly because they don’t know if they do if they haven’t tried it). It’s more illogical and subconscious than a superficial sexual preference. Heterosexual men’s deep-seated aversion to this form of sex-play is mostly the result of what society has taught us about sex and gender roles and not what our bodies might enjoy. Whether you like it or not, most of the time there really is way more than just you and your partner(s) in the bedroom.
The Dish 