Or are healthy, well-adjusted men just more likely to end up married?

Yes thats true. Also if you live alone and slip in shower there might be no one to notice and help you etc. There is no easy answers with anything to do with sociology.

> Men in marriage live longer and healthier lives on average to men in long-term relationships and to single men.

What a blanket statement. Does that apply to every married man, without fail? What about women? Do married women live longer and healthier lives too? If so, why do they file for divorce?

>Does that apply to every married man, without fail?

Not sure if you understand what average means.

If average couple has 1.7 children I can assure you you wont be able to find 0.7 of a child enrolled to any preschool.