If you find out that's true, and your goal is to have a family someday, ditch him. Going shopping or having girls night out once a month, is not equivalent to a weekends or constant overnighters with the guys. When you become mature you want to share your hobbies with the people you care about most. That's why most the family guys around here have K5s or burbs instead of trucks, so they can share their hobby with the people they care about. I know more than one couple that has gotten divorced because she was suppose to stay home with the kiddies while he goes off to play. It doesn't last. In our house, we share as many hobbies as possible including hunting, fishing, fourwheeling, and more. There a reason so many people say the family that plays together, stays together. If you aren't interested in something and he is or vice versa, then fine do a little of it on your own, but if you try it and like it and he wants you to stay home, tough for him. I'd say he can get over it, or he can get over you.
OK, I'm done ranting, heck this might not even apply to her. Disclaimer: I know everyone is different, some wives want to be June Cleaver married to a guy that never comes home, to each his own.
BTW, riding on the back does stink and is not very safe, esp. if the person sitting in the back weighs close to the same or more than the person in front. I wouldn't get a fourwheeler though unless I knew I liked riding one already. Maybe you should try renting one for a weekend. That way you can find out if you like it, find out if you like riding together, and know how much money and time you're willing to invest, and what kind of fourwheeler you might want to buy.