I’d say in so-called loveless marriages you have to be sure that you’re still friends who have each others backs at least. I don’t think it is necessarily always worth rushing to split, and you’ll know when it’s time to split because you’d rather live in just about any set-up, never mind seeing your kids sharing Christmas and some reduced circumstances, than carry on as their spouse.
The difficult bit is a lot of people don’t manage to still treat one another decently and with respect while acknowledging that there is no longer any romantic connection. They’re at war within their marriage. That’s as bad for kids if not worse than an amicable separation (although that can be hard to achieve eh?)
Mutual regard, shared goals and a commitment to raising your family as married parents can replace the attraction/desire but it’d be hard to have those things if you simply can’t stand one another.
Being open about how you both feel and being alert to the risks (ie. not putting yourself in circumstances where you might be tempted to have a wee graze at the greener grass) is vital. Also depends what being with someone you wouldn’t pick now does to your mental health, self-worth etc.
Also worth remembering that marriages can hit bad patches. So if it’s a sense of dissatisfaction, feeling down about your life choices etc, rather than being the victim of abuse, say, it can be hard to know when to call it a day, it might be a case of waiting it out to see if things improve? Only the people involved know what they can tolerate.
I always ask the lottery question. If you dropped £30 million on Friday night, what would you/he do? I wonder how many couples would pass!