I've been lurking here for a while, here's my contribution to the thread. There's more from my gym but these are the most ridiculous:
One day I was doing squats in the rack, and between sets a trainer comes up to me and says "You know, you probably shouldn't be doing squats like that if you don't want man legs. You should do your squats in the smith machine." I replied that given the fact that I'm a girl, I can do the same exercises as guys and still not look like them. He replied "Well yes of course of course... but you don't want to overdevelop your quads. Girls should squat differently to build their hamstrings and glutes." At this point I didn't bother saying that the lowbar squat is hamstring and glute dominant anyway... I just let the awkward silence speak for itself.
Today I was resting from the overhead press and I had two 0.5 kg plates on the bar. Another trainer and his client come up to me and say "Are those yours?" I nod. They start laughing hysterically. "They're so small! They're tiny! That's so funny! What tiny little weights!!!!!" They continue laughing as they walk away, and the trainer tells his client "She could just use collars, why such tiny weights?" And his client replies "Girls don't think of these things." Which is funny because I'm pretty sure the olympic collars they are talking about weigh 2.5 kg each. I proceeded to use my annoyance to fuel my deadlifts while they side-stared me awkwardly because my plates weren't so small anymore.
As a side note, most of the other patrons at my gym are cool and respectful, it's usually the trainers who come up with this crazy stuff for some reason.