I'd guess that among people that "like" rollercoasters, it'd be fairly evenly split between male and female. Among people that are "enthusiasts," yeah, there seem to be more males. But I think that's true of a lot of groups of enthusiasts. You also see a lot more male trekkies, video gamers, role playing gamers, etc.
As far as hobbies, I think women tend to be more interested in craft type hobbies because it's more tangible. I know when I make a craft, I like that I have a physical object to look at and or use in a practical way. Something to show for the time that I put into it. I like rollercoasters, but it doesn't give me any kind of sense of accomplishment.
Physically, I don't think there's anything about giving birth that affects your ability to ride rollercoasters. I think it's more about age when it comes to the physical aspect. I'm finding that I black out on coasters a lot more than I did ten years ago. I don't think that has anything to do with giving birth, I'm just getting older.