I'm always asked this question by english girls. Why I don't know but given I lie and sometimes say yes sometimes no but have a gf, sometimes say I'm single, sometimes widowed, I have noticed a slight change in attitude by the girls.
I've found that if I say I'm married they are a lot more relaxed and the service is slightly better due to them being more relaxed. Saying I'm widowed tends to get the GFE more readily, but saying I'm single or have a gf I would say on average has tended to result in less enjoyable punts. I'm talking about english girls here, foreign girls that I've seen, and it's not many, never ask.
I can only guess why saying I'm single isn't the best idea and that is because maybe the girls think single guys are more likely to want to date them and pester them if they give a really good workout with a guy and hence value for money. Married men conversely are one assumes more likely to want to be forgotten by the hooker, and forget the hooker once they get back to the family.
One thing puzzles me though. Surely hookers must know we punters lie about everything? Surely they can't be that stupid to think we believe the bollocks they tell us and then will tell them the truth about ourselves?