It isnt that you are "more fertile" but you have no idea when your cycle will restart. I was breastfeeding full time, waited my 6 weeks and now have two that are 10 1/2 months apart. That being said, the "no sex" rule is about healing and risk of infection as well as hemmorage. I have seen someone end up in an ER with uncontrollable bleeding and another end up with a terrible infection that almost cost them their uterus. I have also seen a few with just terrible infections.
Not to mention, ur the most fertile after u give birth, unless u want to have kids back to back..
Yes risk for infection is higher and you need time to heal. Also your cervix needs time to close. You will bleed and pass tissue after giving birth, and me personally, I would not want my husband to see that coming out of me.
The body needs to heal and the risk of infection is high. Do not valve sex until your doctor has given you the all clear.