What happens if a virgin girl has sex with a virgin boy?

I’ll assume you’re talking about pre-adult, pubescent teens and not pre-adolescent children or younger.

1. They both have a partnered sexual experience.
2. Hopefully that experience is mutual, informed, consensual, pleasurable, respectful, protected (against STDs, STIs, & pregnancy), and enlightening.
3. They are no longer “virgins” (however you define that, which isn’t as easy as you might think).
4. Whatever they want to happen (mostly) happens.

That’s all I’ve got. : – /