Yeah - I've waffled back and forth on it, too, honestly. But for the Snape who 'lives' in my head, it's more "realistic" (heh) that he hasn't had sex.
Not for any romantic reason or anything like that. I just think he has trust issues miles wide and fathoms deep. Getting naked - even partially so for a seedy shag in a dark alley or something - requires a certain degree of trust and implies a certain degree of vulnerability, no matter how temporary. I don't see him as letting anyone have that "power" over him, the whole control-freak thing.
I also imagine that he has just enough arrogant pride not to "sink" to the sort of one-offs that he might get in a seedy bar or other random place, he'd resist being "used" in turn in such situations, and I mostly don't imagine him being willing to sink to rape (unless I'm reading some yummy non-con, then anything goes! :p)
But. Yes. I know I'm in the minority with the whole Virgin!Snape thing - in fact, I know some people who are as squicked by that as I am by the mpreg, so definitely a 'to each his own' situation. Me, I LOVE it. (laugh)
Probably my favourite thing about the whole Snarry fandom is that there's plenty of room for all of us even despite all our broad range of tastes and various quirks!