It’s always a bad idea to move in with your partner’s parents if you don’t absolutely have to. I think teens and early 20’s is too young to move in with a partner period but I also met my husband when I was 17 and moved in with him almost right away and thats been working for the last 18 years. I still think we’re the exception to the rule though and it’s not exactly a healthy way to start your adult life.
You pay less taxes, its easier to get a loan (if you both have good credit), you automatically have all the rights to know about their health in an emergency situation, whereas a girlfriend/boyfriend needs to go through extra steps, some of which are impossible in an emergency. Some people also view marriage as a very religious thing and so that part of it is a big deal to them.