My boyfriend’s dad showed me a photo of his penis and begged me not to tell my boyfriend. What do I do?

I (25F) have been with my boyfriend (27M) for almost 3 years. He has a complicated relationship with his biological father. They didn’t speak for years and only recently started trying to rebuild things, although they still argue a lot.

A few days ago we were all at his dad’s house waiting for a pizza delivery. When the pizza arrived, my boyfriend went downstairs to get it and I stayed in the living room with his father.

Out of nowhere, his dad pulled out his phone and showed me a picture of his penis. I immediately looked away and asked him what the hell he was doing. He laughed nervously and told me not to tell my boyfriend because it would “destroy” the progress they’ve made in rebuilding their relationship.

I left shortly after and haven’t wanted to see him since. The problem is that my boyfriend is actually happy that things are improving between them, and I know telling him could completely blow everything up.

At the same time, I feel sick keeping this secret and I don’t feel comfortable being around his father ever again.

What do I do? Do I tell my boyfriend and risk ruining their relationship, or keep it to myself and just avoid his dad from now on?