Board Thread:Spoilers!/@comment-26159109-20160117104058/@comment-25926288-20160117143651

Robin had sex with Zelena. He was there and he knows and Zelena comments on his bad skills. Guess Zelena gets around with the love-making, while Regina had Graham.

So, Zelena is certian that Robin is the father and if Robin wasn't the father, she would have said so and they would have let her leave.