libido

So first have you talked to a therapist/medical professional yet? It might be there is something leading to this that they could have insight in.

But there is also the case that this could be perfectly normal for you. Everyone has different sex drives, and I've met some people who basically have no drive at all.

But if your sex drive is different than that of your SO this might pose problems, or lead to a need to discuss other options for them to find release when you aren't in the mood.