I'm still trying to process the thought of a dude NOT liking women in hot outfits that look like your favorite fictional characters. What the hell!?
For the same reason why people don't want to see Rule 34 pictures of their favorite childhood cartoons. Cartoony and 2D characters were never designed with the limitations of our world in mind so even the best cosplayers can never hope to capture the magic of those characters. 3D character models and real people can never look as good their 2D counterparts. Can you imagine what Super Saiyan 3 Goku would look if there was a live action movie adaptation of DBZ?
On top of the fact that some people oppose cosplaying as a whole, regardless of gender of the cosplayer, there is also another thing to consider: many if not most female video characters are designed in a sexist way (a lot of revealing clothing). When you have women dressed up in revealing clothing prancing in an environment where there are a lot more males than females, some people will start to think that they are being "attention whores". Female cosplayers are in a disadvantage because of this.