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What would he do or act like?
Depends on the guy.
But if the girl is really hot what would they do
There is no 100% certain answer on that, most will probably just want to fuck her brain out and keep doing it.
What do you mean by that? What if she is manipulative and high intelligent and hshe drives him crazy with words instead?
Then that's a problem not that she is hot or that she is attached. If you change the parameters you should expect a different result.
What do you mean?