The penis is a wonderful thing. Its amazing how it seems to react. The penis is simply doing what it feels. People have sensors all over there body, be it from touching hands, being tickled or even smelling something.

I've heard of a female that couldn't be around bacon (while its cooking) because it caused her to have an organism - not sure if its true but oh well.

The thing to keep in mind however, is that a erection is normal and natural, I bet the female you are with also gets erect (in the female sense) it just so happens guys can't hide the problem.

Another thing to keep in mind is that just because you have an erection, leaking a lot of pre-cum doesn't mean you want to have sex, it just means this situation is a sexually turn on for you, either that or your body is too "warm" which creates an erection.