Why do men stay with women they never intend to marry?

I see this so often. A man makes a woman pop out all his kids, raise his kids, do all his laundry, cook him dinner every night, help him out with the bills, provide sex, wash his underwear, do everything under the sun for him, including paying his mortgage. BUT, he would never ever propose to her. After the kids are nearly grown and being together for 10+ years, he breaks up with her to get married to someone else (he had known for just 6 months). Clearly he wouldn't marry her because she isn't "the one", she was just a place holder till the woman of his dreams arrived. But my question is... why do men have kids with women who they never intend on marrying?

Why do men stay with women they never intend to marry?
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