I never saw Snape as a virgin. It's actually sounds like a very weird concept.
First, people have sex for different reasons, not only love. Emotional frustration (he definitely had it) - to get distracted from his love tragedy. Experiments - just at least know what they are losing. Alcohol ending in regrettable (or not so) connections. Excluding physical relations from one's life completely is actually not so common in general and should fall under very specific individual state of mind. We can argue Snape was exactly such individual, but it's still not the first thing one would assume.
Remember it's a book for children, so the fact that we don't have any hints on Snape's sexual life may be not because he didn't have any, but because his experience was not appropriate to mention in the contexts of such books.
Also we should take into consideration the social group he was in. His school circle was all the teenagers with very loose moral behavior. He didn't have much of other communication and he desperately wanted to fit in. After all, he acted with Lily not the way he acted in his common life, he didn't care much about his morality when she was not near to judge him.
"No - listen, I didn't mean - "
" - to call me Mudblood? But you call everyone of my birth Mudblood, Severus. Why should I be any different?"
I totally see him experimenting as a young man and being encouraged by the behavior of his school "friends" if nothing else.