Unprotected sex with someone you revere and respect?

Assuming he didn't know he was John's father if I remember correctly... this seems extremely unprofessional and disrespectful. This could be seen as an ultimate act of betrayal to John - who he supposedly had a lot of respect for - let alone could have jeopardized the entire future.

It also opens an unnecessary time loop can of worms, though obviously it was a great emotional payoff given the development of those two characters.