Confusion about Mormonism and marriage

Okay I’m not trying to bash Mormons in this, I’m just venting. I married into a Mormon family. My husband was raised by the most devout Mormons on the planet (I’m talking following ALL the rules). During high school he told his parents that he doesn’t want to pursue it any further since they pushed it on him way too much. His other siblings, however, are still Mormon and have gone on missions, etc. My main question is why do a lot of Mormons get married SO fast?? My brother in law got married right when he got back from his mission after 3 months of knowing this girl. Low and behold, they got divorced 2 years later because he was unhappy. Now he has a new gf and they’ve known each other for ~10 months which is better but he just bought the ring and he’s proposing next month...she is super controlling and honestly kind of crazy. They’re both in the honeymoon phase and I just think it’s so stupid.

She is so mad that he hasn’t proposed yet and she’s always posting on her social media how perfect of a future wife she is and he needs to put a ring on it ASAP. Not tasteful at all. I just don’t get the rush. Is it because they can’t wait to have sex?? That’s another thing I don’t get honestly. I believe in God and have faith but I had sex before marriage and I believe you should “test drive” a car before you buy it. In my opinion, sex is important in a relationship but it’s not everything, of course. Idk it just makes me frustrated because I don’t want him to rush into it again and see that she’s actually not the one. There’s a lot of things I don’t get about the religion but this is a big one.