Your comment does not provide "much substance" either. But allow me to give you my point of view. This is why I am against it.
Most selfishly, I'm worried that I'm going to be inconvenienced by this. I have a tight schedule and a routine and everything works perfectly right now. I'm worried that they will implement women's only hours during a time I visit and throw my schedule out of order. It could mean no more gym time for me that day.
I'm worried that it's going to make the gym even more crowded. It's already extremely packed at peak hours.
If the issue is the girl's religion, then too bad. Welcome to the West, we're a secular society that treats women equally. If you want to keep your backwards religion that treats women as second-class citizens that need to be isolated and covered-up, then keep it to yourself and don't impose it on the rest of us.
If the issue is that girls are intimidated, then I suggest they actually give the weights section a try and talk to some of the guys there. In my six years at the gym, I have never seen any sexual harassment of any form. This is a school gym, we're above that. You'd be surprised at how cool these people are. I was a skinny and underweight guy, and I thought so many of these people were such douchebags. That was until I talked to them and they helped me out, either by spotting me or giving me tips. The vast majority of people don't care about others. They go in, do their stuff, and get out. I realize it's just my experience, but after all these years, I'm pretty certain it's not uniquely mine. I haven't heard of any sexual harassment issues at the gym.
If they're still intimidated, there are other ways to promote and encourage them that don't involve segregation. There are trainers and classes. There are groups. Talk to McGill Athletics about it.
Finally, my most cynical point. Please don't discredit the rest of the arguments based on this point alone, and maybe spare me the downvotes. It's only my perspective. I'm getting the feeling that girls are not equal. They are weak and soft. They are special little snowflakes that we have to protect. Yes, you too can do science. Yes, you too can do math. Yes, you are beautiful no matter what shape you are. The big men at the gym scare you? Aw, that's ok, we'll just tell them to go away so you can exercise in peace. Aw, the mean scientist's shirt offends you? We'll just attack him until we make him cry. I know, that's awful. But it's the feeling I'm starting to get.