Basically, I closed this girl that I have been very interested in. She was making a lot of the initiative. She definitely wanted to do this with me. We hung out with a friend of mine then me and her went to my house. We hung out in my room and messed around, but really didn't do much since my parents were home. So we decided to leave. I drove for awhile trying to think of a way to move into sex, when all of a sudden she yelled "SEX" and hit the roof of my truck. She then explained that whenever a car passes by with a headlight or taillight out you yell sex and hit the roof of the car, then the other person has to take off an article of clothing. I thought this was cute, so I played along. She ended up getting 4 and i got 2. Shoes and socks didn't count. I told her that I was going to park and take the rest off there, she agreed. We parked and enjoyed each other.

Now the thing I am having a problem with is how to make this a trend, something we can do at our leisure rather than just in the heat of the moment. I messed up once before with a girl by trying to hangout with her solely for sex.

I would consider a relationship with this girl however I don't think a relationship would work with us at the moment. I would really like to, at least, continue the sex.
Any ideas?