Another option is to just start making a habit of it.
Work out which of your mates won't ever care about how rude your conversations go, and get comfortable talking about it with them first then progress from there, most guys are never going to care, and then after that move to your platonic female friends, then to potentials.
I am guessing that you are having problems about talking about sex in general, not just talking dirty. Because well, lets face it, it might not really be something you want to do with your male friends.