It really has nothing to do with tanyone judging how well you attract the opposite sex or anything like that. One, people are just in general curious or critical of something against a societal norm. Let's face it...being divorced is 'normal'...not ever marrying or not being married later in life is not.
But there are other reasons, marriage represents certain values that people like to see in others (especially for a guy). Responsibility, stability, dependability. In the major corporate business world it is not uncommon for someone without a wedding ring to be viewed differently than someone with one. (Of course, this is a general statement and I'm sure there are unmarried men in the major corporate world that do just fine.) Nevertheless, there are judgment associates.
Now, maybe...people ask you why you are not married because they feel you are all these "things" (Responsible, stable, dependable). Maybe they are curious. Or maybe they are making inferences, like, "seems" like a good guy...but maybe has commitment issues, isn't interested in others, or has some other weird habit we don't know about."
That's just how it is and people don't do it really intentionally. It's more of a Fruedian thing. Because let's face it...there are plenty of married men and women, more than we would like to believe...that are 'happily married' and cheating on their significant other. There are others that are unhappily married. And there are still others who disagree with everyone's 'new age' cynical view on love and divorce and are HAPPILY married...no one is cheating on anyone else...and they have a lifelong friendship with the other person.