I created a solo adventure for her one day, and we sat down to play. After she stopped hitting on the DM, ahem, she finally started to see the game differently. I then started a campaign with a few friends and she joined in.
That was about 14 years ago. We just played yet another session of DnD together last night.
Not saying this will be your experience, but invite her into that world and see what she thinks. Don't overdo it. Whether she joins in the fun or not, just make sure to spend as much time with her as you do gaming (moderation in all things, heh) and things will probably work out just fine.
And both of them are just hobbies. They're no different than poker night, watching TV shows, model building, putting together puzzles, knitting, collecting penguin sculptures, etc.
Looking over at my husband's computer desk, I see that he's got toys scattered all over it. I mean real toys, like transformers, a jar of putty, a poseable viking figure, etc. I bought him half of the things up there. I get him a transformer of some sort each year for Christmas. He does not actually play with them (at least, not when I'm looking), but he gets joy out of having them, and I like being able to contribute to that silliness. Growing up is overrated.
There is no real truth to that saying "opposites attract". Think about it.
On the other hand, if your partner is basically a carbon-copy of you, liking all the things you like, then your partnership doesn't enrich either of you very much. Might as well date yourself.
For two people who are very much in love with themselves, this could be a great solution!
Growing up is overrated.
Responding to the OP without reading the rest of the thread (sorry) ...
You guys need to work this out between you _before_ you get engaged. To me, getting engaged then married means "I totally love you and accept you exactly the way you are." It's extremely unfair to get married and then tell your spouse "Now I need you to change." If you think this could happen and you both feel strongly on the matter, work it out now or it could be a disaster.
As for growing up and gaming, I'm 40, happily married, two kids, and I'd love to game if I had time but I don't. My wife thought it was a pretty weird activity, but since she trusts my judgment she thought it must have some redeeming quality. Then I ran her on a solo adventure (after we'd been married a couple of years) and she decided it was a pretty fun game.
In truth my motivation to play has gone down over the years, but not because there's anything childish about playing. Part of it is just getting busier. The other part is that when I was a kid I used to play to do cool things I couldn't do in real life. Now that I'm an adult with a decent income I can and do go do cool things in real life. Maybe I can't visit another planet in RL, but I lived in a third world country for a couple of years and that's close enough for all practical purposes.
Anyway hope you saw something useful in my rambling. Good luck.
That's a good point. If you ever DO want to game again though, I can attest that running a game for your kids is a lot of fun. Especially if they are a bit older. I now regularly play Call of Cthulhu with my 12, 15 and 17 year olds. My wife is usually in on these games too.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.