The demerits of anal sex is mainly that it can hurt tremendously if the guy penetrating has no idea what he's doing.
The vagina is a steel-clad cage whereas the anus is a cardboard tube, in terms of how many layers of skin protect it. Therefore, it is VERY easy to rip or tear the anus, which is a) extremely painful, as well as b) extremely dangerous. It can cause massive infections...so be very careful!
Remember to use lots and lots of lube, and it is a good idea to stretch out the anus prior with some anal foreplay before you put an entire penis in. Go slow, and listen to your partner. If they are in pain, stop!
And please, always wear a condom.