I have an ex husband who married an 18 year old, had two boys, she got on drugs, divorce and he raised those boys under 5 by himself and they're on the verge of graduating high school. He did a good job.
Most guys do, when given a chance. With divorce between decent people, I'm for a "time spent" standard: time spent with the child by each parent after the marriage should reflect time spent within the marriage.
As to fairness to parents, it's having them live with the scenario they agreed to when they were together still.
As to the kids, it keeps thing as stable as possible for them.