I once had to march someone out of my house for asking for bareback. Think I gave him quite a shock and he was very embarrassed, and quite apologetic saying he wasn't going to push it, he was only asking. Except he did push it, I said no to him twice. I shouldn't even have had to say no once because he shouldn't have asked!
I think some clients may just ask on the off chance and they're trying their luck but they don't seem to realise once you ask that you then change the tone for the whole booking and it makes the girl feel very uncomfortable. Also they don't think that asking that implies they may have already got it from someone else and could have anything so I definitely don't want to be having sex with them now.