Honestly, there's a lot of different possible causes. For example, a guy may have had bad childhood experiences with women or a woman that caused him to feel anxious around women his whole life. A guy may just be anxious around people in general if he has social anxiety. In most cases, I think guys get anxious around women because they feel the need to be more than who they really are. They feel they need to be extra interesting or funny - more than they normally are - because they worry that she won't like them. It's a fail mindset that only leads to being less interesting and funny. This is why anxiety can me amplified for a lot of guys when they're around a hot girl or a girl they really like. They're afraid of "losing" her and so they put all this pressure on themselves. It's best to just relax and behave as you normally do. Don't try to be something different or something more. Success, again, is about prior preparation. So, if you haven't already become an interesting and funny guy, you're not going to become any more so by thinking you need to. If you already are, then all you have to do is continue doing what you're used to doing. It's no sweat. We are what we do repeatedly. We're creatures of habit. You need to make being interesting and attractive a habit and a lifestyle so that when that girl you like comes around you don't need to put on a show.
I like this. It means youve either succeeded or youve failed. Now is the time to find out. Take a swing and see where you stand. So the question isnt do you have anxiety in front of women, its what are you doing now? That answer is going to determine what happens for you. Are you constantly analysing yourself, are you drooling over women all the time, are you being kind to yourself, are you being engaging? Do you have control?