I mean, you're completely full of shit. There's a lot more men than there used to be but there are still mostly women in yaoi fandoms in general, and there is a lot of spillover into slash fandoms which are at least 95% female. I work for a shop that sells yaoi of all kinds at conventions and it's mostly women who buy it. You seeing mostly men is probably because you hang around mostly men.
I agree that /y/ is more male than it used to be though, I could believe this place is mostly dudes now. It definitely used to be more female but that was before there were social platforms that let you post porn. Before Tumblr the only other place people could post it was Y-Gallery and Furraffinity, so this board was massively more active, and more female.