My partner of 15 years has been messaging his ex from 20 years ago, I shouldn't have read his messages I know but I had a nagging doubt so I looked, it said he'd sent a valentines kiss message and he thought about the way she used to bite his lip! When I confronted him he said it was a joke! I'm hurt and angry as I feel he has betrayed me, he says he hasn't as it was just a message, am I wrong?