There are many ways to ask your wife to pose nude. The first way is, you dont "GET" her to pose. She should want to pose nude.
Reasons for youe wife to pose nude include:
1. She isn't getting any younger. Time passes quickly, before you know it you and your wife will be elderly, and you will wonder what the hell happened to the last 70 years. Believe me, this is true (except I'm only about 50 years in).
2. She's pretty. She's pretty to you. You want to create beautiful photographs.
3. Posing nude is fun. Posing nude for a stranger is exciting. This isn't the same as you shooting your wife nude though, but if you want professional photos, hire a professional photographer.
4 You can fix things in post. Once you take the photos, you can work on them to improve the outcome even more. Make her into art!
Before you ask your wife to pose nude, have some poses in mind, and show her other work. Understand that what works for one model might not work for your wife. This or that pose might show off parts of her body she isn't happy with, other poses might hide that just enough for her to LOVE your work. That's your goal, for HER, your wife, to love the outcome.
If you want to "get" your wife to pose nude, make the outcome something that SHE enjoys. Afterall, She's the boss!
The Art of Boudoir Photography: How to Create Stunning Photographs of Women