How can I get an accurate calorie useage count for this exercise?

I'm on this weight loss program where I'm supposed to be increasing and monitoring my calorie intake and outputs... Anyhow, part of the program involves increasing my physcial activity and I've noticed that I have a tough time getting to the gym... What about sex? I'm much more motivated to have sex. Is it a good workout? I mean, I sometimes see this generic number of calories that is supposedly used in sex, but it really doesn't account for variance in intensity and time. How can I get an accurate count of the calories used for my weight loss program?