So Anonymous asks, "what's the rule on post break-up sex?"
Really simple: DON'T EVER EVER EVER DO IT. duh. I'm going to explain this in a very round-about-backward-deconstructive way. stay with me.
The one thing that I will NEVER understand is people who stay friends with their ex's. Because correct me if I'm wrong, but if you stay friends with your ex, keep in touch, hang out, see each other, talk, email, sms, chat, facebook, whatever, then basically what you're doing is keeping up with all the elements of the relationship that you just "ended" except for the sex. So basically, you've chosen to keep every part of the relationship that is stressful and shitty (ie, having to pretend that you give a shit about each other's problems, spending your free time listening to your partner talk about shit you don't care about, etc) and just elected to remove the one really awesome part - the sex. So now you have to continue to deal with each other's selfish bullshit, but if you're the dude, you don't get to touch her boobs and vagina anymore and if you're the girl, you don't get to... well, have your boobs and vagina touched by the dude anymore. and that makes no sense.
So let's go backwards now. If it was actually possible to remove all the bad shit from a relationship (ie, having to pretend that you give a shit about each other's problems, spending your free time listening to your partner talk about shit you don't care about, etc) and keep all the good stuff (sex and... well, I guess foreplay), if that actually worked, then you would have never broken up in the first place. You would have continued to be "boyfriend and girlfriend" without ever talking, without ever whining or complaining to each other, without ever being disappointed in eachother, without every feeling neglected or rejected, and instead, you would have you just fucked all the time and stayed in your "relationship." This could totally work if it were up to the dude, by the way. No expectations, no "talking" about things, just a lot of penis-in-vagina stuff, mouth-on-boobs stuff, and things along those lines.
So what happens when you bone your ex? Well, you're pretending that you can go back to that place and time when HOW the other person is didn't bother you. When his/her idiocy didn't make you want to ram sharp objects into your face just to avoid dealing with the pain of how stupid they are. When the other person was just a penis or a vagina, a warm body that you thought, "eh, I could love/get along with/tolerate this person."
This is never ever ever ever a good idea. Because as soon as you fuck, you will be reminded of why you broke up. You will lay there in bed (or in the parking lot of Dunkin Donuts, or wherever) remembering how much you love that person's penis/vagina and how much you hate the person that the anatomy is attached to.
Either that or you're one of those chumps who falls in love with everyone you fuck. Which in that case, not only should you not fuck your ex, you should not fuck anyone. Instead, you should go learn how to not be an emotionally crippled freak.