STD causes?

So I went to the doctor after not being able to have a check up in awhile. The results show I have HSV-1 (Herpes). The guy I am dating says he never cheated on me. He is the only guy I’ve slept with since I broke up with my ex.

I was tested when I was with my ex and was negative for all viruses.

In my life time I’ve only slept with 2 men. Any thoughts on what caused this? I am assuming my current boyfriend caused it, but don’t know if he cheated or not.

I caught him masturbating for the first time. He told me he never does because he doesn’t need to since our sex life is healthy. Feels like he is cheating on me and I don’t want to believe it because if it’s true then this would be the 2nd guy I’ve ever slept with who cheated on me.