Other sex seems to imply that they’re are only two. Isn’t that wrong on universities these days? Bad, bad, bad.
Unless it refers to having sex “the other way”, which I’ve heard is all over universities these days, too.
But, seriously, “old people”??? We called them “senior citizens” because “old” was “offensive”!
“Opposite sex” implies an even number. That discriminates against those who perceive an odd number of sexes.
You’re probably supposed to say, “an other sex,” not “the other sex.”