I am not sure if we know whether or not Snow and Charming are married at this time. I think everyone seems to think that Cinderella was pregnant right then, but not necessarily. Rumpel may have just been saying that when they got pregnant the baby was his and not that they were pregnant then. And it was clearly a different day when Cinderella tried to run away and told Thomas that she was pregnant. It could have been years. This could have easily been before Snow and Charming married if they wanted it to be.