I tried being friends with my ex once and all it takes is a drink here and there and you can easily end up back in bed together. It's familiar and you've done it before so the leap into the bed isn't quite as much of a big deal and in a way doesn't seem as wrong because it used to be a normal part of your relationship. I couldn't live with myself and came clean. However, much you might think you're just friends it's dangerous. Of course you can't say you don't trust them because that's just as bad. You're probably in a loose loose situation. Sorry.