Cultural differences too, perhaps? Also, if Evelyn was about to become a mom, maybe she was planning to take time off from school. Luis was working, and an adult, so they may have planned to marry.
I grew up in rural New England, and we even had a few very young pregnant girls (12, 13, 14), AND we had some situations where *very* young couples (like Evelyn) who were not pregnant lived together. (Late 70s/early mid 80s) Now of course, this would be looked at differently, but back then it was more...accepted?
I have a friend who moved in with her boyfriend at 14, they got married a few years later, and they're still together today. She's 50. They're very happy, their kids have graduated college, etc. As a parent myself, I look at this and think OH HELL NO, but I know things were a bit different a few decades ago.....