I think most people can accept this belief while still thinking that there ought to be an out for two people who find themselves deeply unhappy together for a long period of time. Committing to something should very rarely be an absolute thing, and most people (even those who accept and respect marriage as a social institution) don't think that marriage should be one of those things. That's compatible with thinking (as you say in your OP) that most romantic relationships are shallow and not ultimately very meaningful, and wanting something more.

I don't care if they love each other. I care that they're not making me pick sides and often causing me to deal with people I like moving away or quitting common hobbies so they can avoid someone they've gone from loving to loathing.

There are also selfish reasons, including boredom, finding more excitement - but not more depth - with someone else, and festering undiscussed resentment that financial and/or sexual expectations aren't being met.

If one person goes down one of those paths and refuses to even discuss it, there's not much the other person can do.

It's immature to expect a perfect glowing zero-conflict happy ever after. But it's equally immature to stay committed to something that is clearly dead and may be actively harmful to either or both partners - and perhaps the kids too, if there are any.