If he was, I would have to say, there is definite difference between never having had sex and 'being a virgin'. And Severus would fall into the 'never had sex' category. Because, even if he was a virgin, I'm sure he has sexual experience.
I think that Severus snape was a virgin because the only person he probably wanted to it with was Lily because she was his true amor and the person that ever really meant something to him. I also, think that he wuldn't go against Lily because I guess he would feel bad about it like when he called Lily a 'Mudblood'.
There are several reasons for a late virginity. Among them : - to be in amor with someone from very early on and be stuck in that feeling, not allowing anyone else near ; - to have witnesses sexually stressful situations (like a collective rape, for example) and be disgusted with sex. These reasons are not mutually exclusive. They could fit Severus (Lily ; the Death-Eater crimes).
I address this topic in my latest fanfic "The Blood Compact" on FF.net.
At the same time, he may have not been a virgin at all. Sex and amor are two very different things, especially for a man. Hanging out with the Death-Eater crowds, and the likes of Lucius Malfoy, for example, may have not let end up a virgin. Maybe he had even been compelled to do stuff, to test his loyalty.
Only JKR can reply to this question. I'd like to know - after all, she stated that DUmbledore was gay, she could announce that Severus was a virgin ! Nothing would surprise me.
I truly doubt it. I imagine that after Lily spurned him, he went on an "F U" bender (I knew lots of people who did that in high school) and slept w/anyone who seemed willing. As for actually caring for any of those partners, no.
I'd say más likely than not, yes he was a virgin. He wasn't just in amor with Lily, he was totally absorbed por her. She was the reason why he even was alive. That combined with the fact that most people found him to be unattractive and unapproachable I think would keep him a virgin.
But if he wasn't then I think like link said, the most probable time he would have done it would have been between the time Lily broke their friendship and he found out Voldemort's plans for the Potters.