I recently found out my (40F) partner (44M) of 2 years has been chatting with a 20 year old woman (his friend's niece) via social media. Apparently they met one night about 8 month ago at his friends house (he was supposed to be going round to his friends house for a few drinks mid afternoon and ended up staying the night there because he got too drunk). I've never met her, he never mentioned her, it was only eight months later (two days ago) during a fight that it came to the surface (because I looked at his phone and saw a message and asked him about it.)
Yep, I looked at his phone and I should not have done that but there are trust issues from previous dishonesty). I am really pissed with him for forming a secret connection (or whatever it is) with a girl 24 years younger than him and not telling me about. He says it's harmless, that he didn't tell me because he wanted to avoid conflict, and that she had reached out to him and they had exchanged a few pleasantries. BUT he had deleted most of the messages before I saw the last one, so how do I know? Why delete messages if it was innocent. I asked him that and he said he didn't want me to see and feel upset. He maintains it was innocent but I think it sucks.
TL;DR partner deleted message exchange with a much younger woman; maintains that they were harmless messages.