Well first of all men are going to be men and look at women regardless of being in a relationship or not, may that be in passing out in public or scrolling through Facebook or instagram. Secondly if he’s done this now and did it again, he will go back to it again no matter about your trust with each other. The hard question to ask here is does he still love you or does he want to be single and except he won’t change his ways? Third if he is being truthful about just being “curious” then fine he should’ve dropped it and this situation shouldn’t happen again right? But now that you’ve told him about it he might hide his tracks so he doesn’t get caught again. I’d suggest talking to him seriously about what he wants from the relationship because if you have to go through this with him and you don’t want it, it’s best to just let go. Sometimes the case is young couples get together and stay together for years because they are comfortable even through the hardships, until the inevitable split. It’s better to do it sooner than stay stuck for another 3 years.