or they can be friends but because they had sex, the context of their relationship is sexual

you're putting more on the relationship than even the person who told did. clearly everyone sees it as them being friends who had sex a few times but were ultimately friends.

there's nothing to 'deal' with. or rather, what you have to deal with is your own feeling about it which means dick because it had nothing to do with you and still doesn't because YOU WERENT THERE.

has either person expressed more feelings? is anyone acting inappropriately? no?
then the only problem here is you feel silly because you didn't know

you didn't know about something that happened when you weren't around and ceases to exist

EGO

it exists to you because dude is still there but dude hasn't been anything but nice and helpful. you just don't like that someone that close to your girl put his dick in her even when he hasn't indicated in any way he wishes to do that again.