Do I mind? I prefer married men! You are right though, if that is the first question on his mind then he is probably in the wrong place!

They are less likely to fall in love and want to run off with them

exactly right they don't bring any drama, and don't get possessive.

I don't think it's any client's business whether you're married or have an SO.  I never ask a provider that question. However, sometimes they volunteer that info, and I do prefer providers who are married or have an SO, for the same reason many providers prefer married clients: avoiding complications.