I think Andy's got it right......convince her that rafting is like car camping....because it pretty much is....with a little more work. The LUXURIES!!
I got lucky, my wife was a kayaker before we met....so getting her into rafting was pretty damn easy. Now I have a hard time prying the oars out of her hands because she's so excited to get good at it.....which is pretty awesome....just means more kayaking and fishing for me!!!
Don't do a guided trip....get some friends and do a nice, easy float with a few splashes on a hot day....have some drinks and good food. Keeping her warm is HUGE. Throw a few hints in there about how fantastic it would be to do a nice, romantic over-nighter with just the two of you on a full moon night.... and how you'll take some steaks, a good bottle of wine, and a monster paco pad. Done deal.
I don't necessarily agree with the "don't teach her yourself" thing. Maybe not kayaking, but rafting is do-able. I feel better letting my wife learn kayaking skillz from other women, but I/we have no problem on the oars. But maybe you're wife will be perfectly content riding the bow and marvle at her husbands awesomeness in dissecting the river (cause we all know chicks dig good oarsman).