I have been in this situation once before. I was asked to top, I didn't want to and even stated that I was uncomfortable doing it. Needless to say, I did it and could never view him the same after that and it pretty much doomed our relationship. I just couldn't get past the mental picture of him begging to be fucked up the ass and then me being in control of his orgasm. Somehow it really pissed me off and freaked me out, but that is just me.