For no explainable reason i don't want to

I love my girlfriend
We are both virgins
We are both heterosexual
We'll fool around in the sense of i finger her, but when it comes time for her to reciprocate, i stop her.
I don't know why, I just don't want to continue.
I was never sexually abused.
I am for sure heterosexual.
I just don't know what I should do/what is wrong