For the most part there's an allure to doing something illicit. There was a time when it would piss my wife off, but we ended up having a long talk about it and now she helps.

But again, sometimes there are those nights when you need a good wank to get to sleep and she'd already be passed out and I wouldn't want to disturb her.

Othertimes, it's for something specific I can't get during sex - be that a certian stroke or just the oneness of it. Her libedo isn't as high as mine is and so it helps compensate everything.