why

I hate how every conversation I have with a male co-worker or customer always includes the question “are you married” or “do you have a boyfriend” I never know what to say. I feel like saying “why are you asking me that question” or “that is personal” Instead I answer honestly and say “no I am divorced or I am single” . I am an honest person and the truth is what I say. This ALWAYS ens up looking like an open door to the men and they think I am up for grabs or letting them know I am ready willing and able. I have now started to lie and say I have a serious boyfriend or just change the topic. Women do not go around asking male co-workers if they have a girlfriend or if they are married. Why do men feel they can do this to women. It makes me very uncomfortable.
I want to say that it is none of their business and has nothing to do with work. But if I did that i would be viewed as a bitch and hard to get along with.