How long have you been with this girl? I know that back in the day me and the misses used to hump like rabbits every free chance we got.
5 years of marriage later and we still get it on frequently, but we don't have at it every time we're alone. We still have to eat dinner and do laundtry and whot not.

Plus you're a guy. Guys think about sex constantly and once started can't stop.

I've found that women can be in the middle of sex and one little thing (phone call, knock on the door, whatever) happens and she's out in a snap.