That's really the crux of the matter. OH and I were "together" for 10 years before marriage, that wasn't living together, it was basically very long term dating, going on holidays together, etc. I can honestly say we had no surprises at all when we finally got married and started living together in our first home. OH's behaviour/attitude didn't change one bit. It's easy looking back and saying we "could have" got married and/or lived together years earlier, but at the time neither of us were 100% certain and we're the kind of people to need a pretty high degree of certainty before making any decision - we're both definitely more of a "head" person than making decisions by feelings or heart.
We've been together for 35 years now and been through one hell of a lot of challenges including long term illnesses and deaths of parents, OH having incurable cancer, a premature baby in SCBU for several weeks, near bankruptcy, etc., but we work through it all together and 100% support each other.
My previous relationship lasted just 3 months - he was romantic (sang love songs to me that he'd written) and sex was great, but there were just far too many red flags re his behaviour and attitude and it was a massive wrench but I felt I had to end it as I couldn't trust him to be a good long term partner once the initial excitement faded away (which it always does). I know of him through friends and he's had a succession of failed relationships, children with different partners, etc., so it sounds I made the right decision on that one!
I do think people tend to do things far too quickly these days, i.e. having children together, buying/renting a house together, getting married, before they really "know" each other. There's one hell of a difference between romantic/sexual compatibility and the realities of being a parent/homeowner/spouse. I also think too many people put pressure on themselves to grab a partner because they're frightened of not having one, so basically they're grateful for anyone! I really cringe when I see colleagues/family moving in together or having children literally within months of first meeting - there's no way on Earth that they know enough about each other to cope with longer term commitments. (Except for the very rare exception of two "right" people meeting each other, which does occasionally happen). Too many people, women especially, expect/hope their partner will change when they're parents or get married, and all too often, that simply doesn't happen!