Only suggestion possible that I can make with women matters:
Explain to her that if she is a casual smoker, she can damage her lungs more than with homegrown, since the one found on the streets are often imporperly dryed and cured.
You can also benefit from the money it'll make you save. Here's an example:
Where I'm from, 1g = about 10$CAN usually. (let's say 1 an ounce (14g) is about, usually 90-120$CAN)
Just from one indoor plant, you could get an approx 1/2 ounce dry, if you grow properly.
And since seeds over the internet can be found at around... 40$ (for a pack of 10) the diference of money is quite significant.
That's pretty much why I decided to grow a little. I'm not much of a smoker myself, quite the contrairy, but I see the benefit of self-supply.
If worse comes to worse, it's just for the pleasure of taking care of a plant.
(Although, if you're talking about something like a 30 plant growroom, then I aggree with your wife. Anything smaller than 10, I'm for it)
Oh, and don't leave your wife over such a trivial matter. Sure, it'll maybe put a small cold in your relation, but what do you prefer: the temporary high of weed, and of self supply, or the almost permanent high of love with your wife?