Because sex isn't always hot.
"Sex and pizza, they say, are similar. When it's good, it's good. When it's bad, you get it on your shirt." - Mike Birbilgia
Sex. The inevitability of almost EVERY boy-meets-girl (or boy-meets-boy, girl-meets-girl, whatever floats your boat). It's easily one of the most messily fun things we humans get to do, and if you're lucky, you get to have a LOT of it. If you're luckier, you get to have a lot of it with the same person, all the while feeling all mushy and emotional about it. All the while being with someone you'd actually want to have breakfast with the next day, and then lunch, and then you know...maybe a wedding. And if you're sleeping with this special someone, then your sex-life probaaaably looks something like this.