Why would a woman in a relationship not want to/be willing give a bj more often?

Well would you want one of those things in your mouth? I don't like going down either, but if it makes her happy enough, you know, give and take in every relationship (and in one case, there was a ton of give and no take to speak of)

My wife loves it but she has to have had a shower first

Did he give her oral? Some women just don't like it; it's valid.