How do I approach this???

Hey ladies, so me and my fiancé have been together 11 years and have two kids. Last year before our second was born, I went through his phone ( yes I know I shouldn’t have) and saw that he was watching gay porn. When I asked him about it he got extremely defensive and said something happened to him when he was in high school and he doesn’t want to talk about it. Well, 7 months later I’m still thinking about it and now I’m having dreams about it. I don’t want to be selfish and pry to figure out what happened but how do I handle this? It’s bothering me so much now and I don’t want to enter a marriage without me knowing certain things. Should I just leave it alone or ask about it before our wedding next year? He hasn’t given me any information about it specifically, except that he isn’t gay. I’m only asking because it’s causing a lot of panic within me but I also want to be respectful of his wishes