I play devil advocate, they are very young. She just turned 22 or maybe she is still 21. If there were older I would say you should know better. I think when it was just one kid, they just figured they would live together with the one kid, now the 2nd is on the way and it solidifies the family, like she obviously ins't going anywhere so get married.
The only issue I have with her is she shouldn't have tried to cover it up. Just say he hasn't asked you. That way, when you do get engaged you aren't back pedaling on your thoughts about marriage.