Age and admitting erotic thoughts

Let me start by saying I think in most cases it is a good idea to talk to my therapist about any feelings I have toward him, anger or disappointment for example.

Recently I started having intense sexual thoughts about my therapist. It's not romantic at all, purely sexual. Before this I saw him as fatherly.

I'm not sure about bringing this up with him mainly because I'm only 18 and he is 30-40ish. We haven't talked about sex at all or even my dating life (I don't have one hah). It just seems inappropriate to have this conversation with him? I don't want to risk losing him as a therapist because I unwillingly crossed the line (in my thoughts only, I don't "actually " want to have sex with him).

Usually we are very playful. He asked me once if that is how I flirt, but it's not. I'm like that with my grandparents and everyone. I like to joke around a lot and it's been difficult recently because I feel guilty for imaging him in a sexual way. He noticed and asked if I'm ok or upset with him? I lied and said I'm just tired.

Would you talk to him or hope it goes away? Does anyone have experience with this happening with this kind of age gap? He mainly works with ages 13-19 if that matters.