What I don't get about Christianity, I find this illogical, Hope you do?

Please explain how it is that we are all descendants of Adam and Eve. Christians believe that God created Adam from dust and then created Eve from Adam's rib. According to Christians, the two of them hooked up and had two kids: both male. So, unless Eve was doing it with Cain or Able (or both of them), which breaks at least two commandments, there wasn't any procreating going on in the Garden of Eden. Ergo, we are not all descendants of Adam and Eve. So, does this deter your faith or open new doors at all?