As a former pothead, I've had lots of experience with this
pros: for me, marijuana enhances all sensations (touch, taste, etc.) and it fills me with a sense of wonder. So having sex while high can definitely be a treat. Sometimes, it's like having sex for the first time. Also, smoking has acted like an aphrodisiac for me. For some reason, it gets my motor running.
cons: like any drug, it can impair your judgment, and you may end up doing something that you'll regret later. Also, if you smoke too much, you risk getting too stoned to be able to perform, or possibly even having a little bit of a freak out (trust me, this sucks).
If you feel that you eant to experiment with it, I'd be careful to do it with someone that you feel comfortable with.