In Neil Gaiman's "How to Talk to Girls at Parties," what do you think Vic sees at the end of the story that upsets him so much?

At the end of the story, Vic hurries Enn home. Vic is visibly nervous and agitated about what he's just seen.
Although the author is ambiguous about the reason for Vic's angst, the text provides some hints:

We ran then, me and Vic, away from the party and the tourists and the twilight, ran as if a lightning storm was on our heels. . . I held on to a wall, and Vic threw up, hard and long, into the gutter. (Because of what he's seen, Vic is upset enough to throw up). He wiped his mouth. "She wasn't a--" He stopped. (The girl Vic was with, Stella, wasn't human). He shook his head. Then he said, "You know. . . I think there's a thing. When you've gone as far as you dare. And if you go any further, you wouldn't be you anymore? You'd be the person who'd done that? The places you just can't go. . . I think that happened to me tonight." (Does Vic metaphorically or literally mean that he's gone as far as he dares?)

Although this portion of the text is frustrating (perhaps because it's so ambiguous), it's also humorous. On one level, human girls can be so alien in temperament and emotional make-up to boys that they invite curiosity and amusement from their male counterparts. On another level, Gaiman explores how an interplanetary being could actually be so unusual to a human boy that, upon disrobing, she would devastate him. From the text, we can extrapolate that the experience of trying to bed an alien girl was extremely traumatizing for Vic. He was so traumatized by what he saw that he wasn't prepared to go any further. In fact, Stella's emphatic "I am not finished. There is yet more of me" further repels Vic.
So, it appears from the story that Neil Gaiman is posing a distinctly amusing question: if a male teenager thinks that girls are strange, what will he do when he meets a real, interplanetary female alien?