If it is bothering you, you should talk to someone about it regardless of your age. I don't know what level of a therapist you saw so I don't know how well they might understand your problem. You might want to seek someone that familiar in sex related issues/topics (BDSM for example) and is willing to work with someone of your age. You might find that to be difficult without involving your parents/guardians.

In terms of these being intrusive thoughts, I found this site interesting: https://ocdla.com/obsessionalocd

So to some degree, it does depend on where your concern is around your feelings. For many BDSM is an acceptable way of life and for others it is not. This is where the right therapist could help you explore your feelings and thoughts, perhaps figure out what they mean for you and your beliefs.