My ex left me when the children were 4 and 6. No way did I contemplate another relationship. They had already lost one parent, they did not need to "lose" the second parent to being preoccupied with developing a relationship with a stranger to the children.
My two are now adults with grown-up families of their own, and stable, long-term marriages. I do not regret the lack of a partner for myself since 1977, it has been worth it to see my children's steady lives.
But, I do understand that some cannot live without emotional and financial support, so maybe this is the only way forward for them.