Oh goody! I guess I'm special then. I'm 23-years-old and I'm a guy (though, I'll have to check in the mirror later just to be sure
Joking, joking! But in all seriousness, though, there's different reasons why a lot of guys (well, people in general) don't believe in astrology, so I'll list a few here.
1. On the one hand you have the people of a religion that states that astrology is a form of divination and so they don't bother looking more into it since they think it's a tool of the devil's and they don't want to go to hell. I had a friend who was interested in it at first, but he gave up on it after finding out that it's 'bad' (he's a Christian by the way).
2. On the other hand you also have the people (particularly men) who don't believe in astrology because there's no scientific proof of its effects or because they are overly logical and have this 'if I don't see it, I don't believe it' type of mentality. I remember a long time ago I saw a video on Youtube of Richard Dawkins testing out the effects of horoscopes with different people and 'proving' that it's not real. The thing is that he only tested it out using Sun signs only. I don't remember it all in detail since I saw it years ago, but that's all I could remember, but yeah, it also boils down to the fact that some of these same atheists can be pretty stubborn to convince as well since they think that they're right that there is no deity or higher power and that life is a mere coincidence. I used to be an atheist, and honestly before I got into astrology I would've also been a skeptic if someone else tried to convince me of its effects, that is, until I found out for myself.
3. Then you also have the guys who see astrology as a 'feminine' hobby, and so they don't bother looking more into it since they think it'll somehow make them lose their manliness. Yes, this is insecurity to the extreme, and yes, it does make me want to tie myself to a chair while being forced to listen to the meow mix song for 10 hours straight because even that would probably be heavenly compared to dealing with people like that.