Unfortunately, after the bullying he received at school. Jun has an unhealthy relationship with sex. This is why Satoshi is so protective of him (even without knowing the full extent of the abuse.) I jumped forwards by quite a few weeks as well, which I guess I didn't explain very clearly. So for him, it was a merely physical act - which turned out to be much more than that.
Sorry about the late update - I was really busy *sigh*.
Poor Aiba just wanted to see Jun in the flesh to see if he matched his portrait and to see how Sho behaved around him. Sho really frightened the poor thing.
Thanks for reading, my friend *hugs*