I think it's more to do with the cervix closing back up, and wounds healing (the place where the placenta detaches at birth is a pretty big one) and risk of infection. Especially with stitches too. They usually check you at 6 weeks after birth, and they say you shouldn't have sex until you've been checked out by the doctor.
However, my husband and I only lasted two weeks after the birth of our first- or maybe it was three weeks- all I know is we tried it very early on, and it was too uncomfortable, so we tried and succeeded a week later !!!

At my 6 week check, the doctor pronounced me sound as a bell, and actually asked me if I was having enjoyable sex again yet- I guess he's used to seeing people who don't wait the full 6 weeks !!!!!

Do whatever feels right for you. If you feel ready, try it, if it's uncomfortable, stop.
Good luck.