I think a lot of wives are scared because they hop on a bike that's way too much for them. Throw them on a honda trail 50 or a PW80 nd they'll get hooked. At least that's how it was with my wife. I really had to push her to get on a bike. She learned on an XR80 with a clutch, and she didn't like it the first 10 min. After 20 min, she was ready for the TTR-125L.
I was surprised when she told me a month ago "I miss dirt biking, let's plan some rides this year." It came out of nowhere. She's not good, but she can handle a fire road just fine without crashing. She even did a bit of singletrack.
I think the key to wives liking it are:
1) being able to flat-foot (HUGE confidence builder)
2) riding in a place free of distractions. Make sure it's an open place
3) no pressure (or lots of pressure, depending on your wife
)
4) if you can teach them on a bike without a clutch, even better. Noob riders get stressed about what each lever does. If you think about it, there's a lot going on. Left foot shifts down for first, then up from there. Right foot is rear brake. Left lever is clutch, right lever is front brake (opposite a bike). Twist throttle away from you to go forward (not intuitive for noobs), as you accelerate, your body moves back, making you give it MORE throttle. It can be a lot to think about right off the bat.