That is NOT a hard and fast rule and is not universal, but generally that is how it works out (anecdata). How a same-sex/multi-person/trans relationship works out usually, I have no idea and would love some perspectives on it from HN.
Here's how it worked:
A company would hire a bunch of high school or college grads. Young women would get hired as secretaries at a salary of $X. Young men would get hired as salesmen, clerks, etc. at a salary of $X+Y. There would be plenty of time to socialize at the company cafeteria, office parties, and happy hour. Naturally, many of the young men and women would start dating. When they announced they were getting married, they'd get hauled in to the boss's office and told that one of them had to quit. It was then strongly implied that it should be the woman since her salary wasn't going to support both of them and she didn't have much upward mobility. This created openings so you could hire a new crop of young women ready to be paid poorly and marry the next crop of young men.
> Marriage bars provided more opportunity for those whom proponents viewed as "actually" needing employment, such as single women.
as a claimed reason for marriage bars, which is actually quite similar to "firing for the purpose of hiring younger people", though it's being spun as for people's good, not to decrease salary expenses.
But the effects were somewhat pernicious, because as usual people didn't think about second-order effects. Or in some cases first-order ones, like women delaying marriage so they would not lose their job.