Nebula,
Most non-Aspie guys have the same feelings during their first romantic encounters, they just don't talk about it. Sexual situations are frightening until you have a few positive experiences. . It sounds to me that you want to proceed, but you're putting unrealistic expectations on yourself, which is why you're nervous. My advice is to just be honest about your feelings with your prospective partner.
I experienced exactly the same situation with my first girlfriend (I actually physically shook with nervousness the first time we made out). She was interested in me, so she hung-around through the the first few times we were intimate. After two or three encounters, my nerves sorted themselves out and I was a confident pro! Physical intimacy is so enjoyable that most don't remain nervous for long after getting a taste...