Some people just can’t help themselves; despite having an ugly break-up, certain individuals love nothing more than to have sex with their exes. Most sound minded folks like to point out the obvious; if the union ended for a reason, why are these two getting in the sack together again? Not to mention the consequences that can come from hooking up. It’s doubtful that a night of sex resolved any of the past issues the ex-lovers had.
There is a scientific explanation for this type of behaviour. It doesn’t excuse it or condone hooking up, it simply clarifies this reckless sex.