When the guys penis is so small that it doesn't break the girls hymen, thus causing her to still be a technical virgin.
Debbie: So how was it last night?
Susan: I'm a half-a-virgin now.
Debbie: Oh, I'm sorry.
You become 'Half-A-Virgin'... or use the excuse, because you've tried VERY VERY VERY hard to remain pure. But have this one experience when a dick 'may or may not have' entered a place it wasn't supposed to.
Hence, you are 'half'. It was in there, but not long enough for it to take away your virginity.
Person K: So has anyone ever had a PIECE OF YOUR PIE?!
Person A: No. I'm Half-A-Virgin.
Person K: How is that possible?!
Person A: Maybe you should check out UrbanDictionary, to answer that question.