MDMA apparently causes the release of Oxytocin, the chemical that our brains naturally produce to initiate feelings of love, connection, and bonding. Kissing and sex naturally release this chemical, so when you throw MDMA into the mixture, there's bound to be a huge connection with whoever you're with.
As far as sex being bad on MDMA or it being ruined afterwards, it's all rumor... Just because you have sex on E, it doesn't mean you'll never be able to have good sex again unless you're on E. That's just ridiculous lol...