First, she doesn't work outside the home. So any money she spends comes from me anyway. As Ben said, there's no difference between using the ATM and taking it from my wallet. Since I nearly always use the debit card when I have a choice, the only time I need cash is for the few places that don't take plastic. And at those places I'm rarely buying something for more than $20.
If she had "her own" money, and was taking "mine" because she ran out, that might be what you're talking about. But I don't think so. It's like saying that I'm stealing from her because I ate the last piece of cake without asking.