Again, I think that Leo would never have assumed that Regina was Zelena. He would have known about Regina’s birth, as she was born to royalty and publicly presented as such, and he’s capable of math–it would have happened a minimum of 16 months after he met Cora.
Doesn’t make it less sketchy that he was willing to marry Regina after being engaged to her mother, but I don’t think that there’s the slightest reason to believe that he still loved Cora then or that he believed he was marrying the baby that she’d been carrying when they were engaged.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing