the problem is that sometimes women get pregnant during residency (even if they have enough merit badges to do ortho). this can hurt a small program that only takes 2-3 residents per year - the call schedule can become unfairly difficult for the people, male or female, who have to pick up the slack for the chick who got knocked up 👎
yes, it's a bs reason to most people, but it's legit to the residents who have no choice but to suck it up and deal with it.