The content of this post will probably be more along the lines of "why I have to explain to people that I am not getting married" than "why I am not married." Because honestly, not *too* many people I know are married or engaged, and they are typically either a) older than I am or b) more likely to be married or engaged because of religious or cultural pressures that don't apply to me.
Yet in the past, say, three months, probably about 6 of my close friends have asked in so many words if I/we am/are "thinking about" marriage or "future plans." (In fact, one of my boyfriend's friends asked him specifically if *I* wanted to get married, with the implication that I, woman that I am, would certainly start putting the screws on at this point!) I've been with my boyfriend for 4.5 years, 2 of which we were living together full-time, so on the one hand it's not a crazy question; on the other, neither of us has indicated to the aforementioned people that we are anything but happy and stable in our current situation. I generally explain, when asked, that we've talked about it, and it will probably happen at some point, but not in the near future - there are no pressing reasons (religious, cultural, psychological, social, familial, economic, legal, etc.) to do so. Both of us are generally wary of what marriage as a social institution has come to represent (that's perhaps material for a whole other post). And our current living quarters are certainly too small for the new appliances that one generally gets as gifts. :)
I can't really explain the sudden burst of questioning other than by a convergence of two factors - the first round of marriages/engagements is starting among people we know; and the drive for narrative closure, in the full novelistic sense of "then they got married and their stories ended." Hell, we're both in grad school, we'll probably both go on to be academics, all that's left is for us to get married and then people can stop wondering about us ever again.
I'm not very big on Valentine's Day as the Hallmark-sponsored holiday that drives men and women to ridiculous measures. So instead I'll just send everyone (married, single, living in sin, etc.) wishes for a happy evening.