I don't think it's a guy thing... I'm a girl and I dont care much for mares either. It's a personal opinion, much the same as breeds, colours, ect. Look at how many people think TB's are complete lunatics.
When I first landed in Ireland I was amazed by the amount of people who had something against chesnut mares in particular - they called them red headed witches and said their colour matched their personality - fiery. Of course theres no truth in it, a horse's personality/temperment wont be down to their colour, but it's an opinion and I leave them to it.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.