Well, they referred to the baby as a "lttle cowboy", so it was likely Steve's.
And with Lord Lachlan gone now, his nuclear plant will likely go too, and the people will become fertile again.

Maybe they don't know it was Steve's, but when you are deeply religious, you bend reality to fit your beliefs.

They you weren't paying attention.
They clearly said that he was in the hospital. After going for the seventh orgasm.
kali_demidarkness wrote:
"It could have been the cop's baby. It sounded like he was new to the area and would therefore have viable sperm. I was also under the impression that she had so much sex with him in order to increase her chances of conception. I believe she killed him with the scarf too, as maybe her own personal sacrifice to ensure she would become pregnant with his child."