I know quite a few men who settled.
One was in a volatile and passionate relationship for a few years where he really loved the girl, and she was unfaithful in the end, they split. He started dating a woman who absolutely adored him, you could clearly see she felt enormously lucky that he even looked her way. He liked her and appreciated her, but never adored her.
They are married now and she still visibly works so so hard to be the perfect wife for him. He is nice enough but you can tell his heart isn't particularly in it. If he met a woman who tugged his heartstrings he'd be unfaithful in the bat of an eye, I'm sure. But he feels safe and secure with this lady and is, I think, still licking his wounds re his ex, so that's what he needs.
Same with another chap I know. Was dumped by his uni gf / love of his life when he was almost 30, he panicked and proposed to the next girl within 8 months of meeting her, married within 18 months. He's now earnestly checking boxes with her but you can sort of tell he is running out of adrenalin and may soon realise his error.
I can think of a few more men, probably 50% of the men in couples that I know, who probably married for musical chairs reasons as described above. Everyone starts having weddings and then babies and some men feel they need to give their partner "their turn" etc.
My ex never liked me much but I wanted to shag him and was a pretty girl, so he talked himself into marrying me, which is definitely a type of settling.
It happens all the time.
Marriage has only recently become something that we turn to for romance and personal fulfilment, after all. Up until very recently, there was one kind of marriage, and it's what we now call "settling".