What is it about ex-sex (is that an actual word...does it need the hyphen?) that's so appealing? Is it because you know you really shouldn't? Because it's a little bit naughty and a little bit nice? I think it's that it's a little bit forbidden.
I think it's the fact that it's a little bit like an affair; so long as neither of you actually want to get back together then it can be quite thrilling because you know it's bad but not as bad as cheating. You can have a one night stand without increasing your numbers - plus, you know where everything goes!
I'd liken it to realising you fit into an old pair of jeans; you've outgrown them, so you pack them away at the back of your wardrobe - JUST IN CASE. Then one fine and skinny day, you stumble across them and suddenly they're like new again, except you've already worn them in.
You think they made you look hot (when in reality they were too tight and stunted your growth) so you throw them on forgetting that rather than highlighting your assets they exposed all your flaws. Even knowing friends will give you sideways glances and remind you that there was a reason you flung them to the back of the wardrobe doesn't stop you...but if you really didn't want them, or you really believed you would never wear them again you would have thrown them in the Good Sammys bin, cut them up and made them into shorts or lent them to a skinnier friend (NOT RECOMMENDED).
That's the thing about past trends; they just keep on coming back. Even if they weren't flattering the first time and didn't bring out the best in you, you'll most likely forget and put them back in rotation for another season...