It is sad and no-one finds it sadder than me. There was a thread on here a while back (can't find it now but I took note of a few answers) - the question being will a man be with a woman for a few years, have her meet the family etc and still only regard her as someone you have sex with - and how long do you stay with someone just for sex?
One of the answers being yes, to be blunt, lots of men could happily do this indefinitely, and do. The passing of time and doing stuff together doesn't necessarily change much in the male brain. Don't assume that for a man intimacy and the passing of time have made the relationship any more deep, meaningful, vulnerable etc. It may feel self-evident to you that it should be the case, but for a guy it ain't necessarily so.
And then add into the mix, all the men (and women too of-course) that are cheating - dating sites are full of them. That includes the happily married, engaged in family life ones. Men want to have sex with more than one woman and if they had the opportunity and guarantee of not being caught, they would.
Then there are the couples who are settling for someone - how depressing is that.... being the "she'll do" woman? And they're not going to tell you that they're settling, so how do you know if they are or aren't?
I just don't know how to navigate it all when we're so fundamentally different.
Anyway, I'm derailing your thread @CantAskAnyoneElse , apologies. Just needed to let it out.