Actually she likes to be tied up. Do you make your wife go out braless, I love the attention my wife gets when she goes out braless, her nipples are always hard like your wife's.
I don't make her do anything, but I strongly request that she go without bra every now and then. She's only done it a few times when we were younger. Not recently, but we're at a time in our lives where she'd be more likely to in the right setting - i.e. an adult club, at an adults only resort, somewhere far away from home, etc. The next time it's the right time, right place, I'm sure it will happen again.