I've always preferred not to know and I've never outright asked a guy, (although quite a few offer it upfront).  That's my personal preference, because if he is married I'd rather not know. (rather not explain my views on that) I don't discriminate though, to each their own.  

Single men are nice, because they don't have to sneak around and it's easier to get together (usually).  But then again, single men can also become attached to the provider, and try to take it to a place the girl doesn't want to go.  

Married men - well, they are already in a relationship and it could be a big problem if a wife found out.  But then again, a married man is going to go home to her, and in most cases, you don't have to worry about them becoming *too* attached to you. I think that explains the ads I've seen state "Married Men Only."  

There are many other types of guys too, like widowed and divorced, and all single men or all married men don't fit the scenarios I mentioned, but that's generally what I think is the case.