I know you didn't point this to me, but I will throw my two cents in. ;) I don't mean to be argumentative, but apparently it didn't matter at all to the parents when they shacked up and had them in the first place. Michael's girlfriend could have kept from getting pregnant had they not wanted to.
If I recall, the parents were hoping that Michael would move on in his life, and if he met someone, were hopeful that he'd want to divorce Terri so he could get remarried.
The tramp got pregnant because she didn't give a damn, and they both don't care about the media attention of him being an adulterer and her a w&%*&.