Congrats, man...truly. My guess is you are afraid you are going to fuck things up somehow...though it's not clear to me the "what" you think you need to spill the beans on. Outside of telling her all this turns you on (I imagine this is already obvious to her), what more do you need to say at this point? She's enjoys it (gets off) and you do too, otherwise you wouldn't be here
That's all that's required, IMO, to just enjoy the ride and see if this relationship really goes anywhere. As you get closer, my guess is (unless you REALLY lack social skills...which I hope is not the case
) the specifics, level of enjoyment, or what have you can simply unfold in the context of friendly chats, pillow talk, or playful dinner conversation. If you turn your 'fetish' in some external object that you need to present to her for discussion or approval, then I think you are headed down the wrong path...and you may not be as comfortable as you think with your own predilictions. Here's hoping you are
postscript:
Are you worried that you need to confess that you have 2 tetrabytes of facesitting, headscissor, or f/m wrestling clips stored on your harddrives...or something along these lines?