Because he goes home at night and has stuff to do thats not work and he isnt jealous of you for having that. Worst managers I ever worked for are divorced (and to an extent single) and hate seeing happily married employees

Not the poster, but I assume the idea is because a company of only young people often don't consider or understand the limitations/responsibilities of family life. Once you have kids, suddenly "corporate weekend trips!" and "hackathon all-nighters!" and even "beers after work on Friday!" become complicated commitments that require adjustments from the whole family. With a couple family-havers in the mix there is usually more acceptance of the work-life divide, or even that one exists (or should).