I wanted to write a long post originally, but then decided I'd just give you a couple of examples based on what I've seen irl, details somewhat changed, obviously.
A guy goes on vacation and picks up a divorced single mom who is cute and very easy. Somehow she gets pregnant and he takes her and the kid back home. He doesn't actually know much about her and her character besides that she's good in bed (or so he thinks). He later finds out that she is lazy, low class, entitled and has substance abuse problems.
Case 2. A good Christian girl keeps herself for marriage and doesn't date much. She meets a guy who is also a believer from a decent Christian middle class family and falls in love. The family wine and dine her and that makes her blind to the fact that the guy in question is a school dropout, can't hold a job and gets a burnout the moment he has to exert himself. After the wedding she finds out that he expects her to keep working even after the children come, prefers to work part-time himself but won't lift a finger to help with the housework.
Case 3. A Christian guy meets a Christian girl who is pretty, good-natured and a virgin. He marries her and then finds out that she has a princess complex, doesn't cook, doesn't clean but has a taste for luxury.
Add to these an example of a guy who is an eternal student because he doesn't want to work and gets pain in his head/back from working a normal working week, a party girl who had like 100 men in her bed but is now "reformed" and wishes for a good Christian husband (but still likes to party), a guy who is 30 but expects his parents to take care of him and a girl who says she doesn't want children because her career is more important.
I could go on, but I think you get my point. If you wish to have a good marriage and a normal family life avoid these types like the plague. Let them marry each other...