I don’t think Regina would have left SB to have some random one night stand just to have a child, she did like to sleep around, but the world outside of SB was a big unknown for her, and prob took her a while to realize she could cross the line and come back, that knowledge was not just automatically there, she had to figure out lots of things about what the curse actually did. Her not getting pregnant in SB I can understand, but FTL is different, so I think it’s highly possible that Cora made her infertile, or she was just naturally infertile. Which would make her feel even more out of place with Leopold, just one more thing she couldn’t give him, making her feel useless and not wanted.
But! I do think it would be an awesome twist if we find out she did indeed have a child with Leopold, and she thought it died. But really Rumple took it and stashed it away someplace.. Would be really cool if it turned out to be Peter Pan, but this I know is a BIG LONGSHOT!!