Is it wrong or crappy of me to join the marines when I have a baby on the way? Would you wait for birth?

You have listed some excellent reason for wanting to join. Also, it is great to hear that you want to join the finest fighting force the world has ever known. The most immediate benefit you will receive will be the fact that the birth of your child will be paid for by the military, and trust me, that is a HUGE deal. Good luck to you bro.