Many LD women think to themselves "If I looked like Angelina Jolie, then YES I would be horny all the time".
This is sort of true, too. Because women are inherently vain and they (mistakenly) believe that sexual attractiveness is only reserved for those who are "perfect" or nearly perfect. So women themselves actually would in many cases feel more sexual if they felt physically sexier.
If your wife feels she is out of shape or has body issues due to pregnancy etc, this will definitely be an issue for her and there isn't much you can do to change her mind about it. I just wanted to acknowlede to you and to other men out there that YES this is a problem within women's mind's. She could be sooooo sexy if she were only confident, but in her mind, to be confident, she would have to look different than she does. Very difficult for a woman to shake those feelings.
However...in the case of asking her to strip...there is a bit of a way around it.
Most women have at least one feature they feel is "good enough" or "sexy enough", even the most unconfident woman. Some women feel that way about their boobs, some about their legs or their hiney, some feel that no matter how out of shape they feel in other areas of their body at least their "kitty" still looks good.
If your wife has even one area of her body that she feels confident about, then you can stage the strip show about only what she feels confident enough to show off to you, versus getting full on naked.
So just taking boobs as an example....if she feels confident that YOU find her boobs very very sexy, then here's how you modify my suggestion above:
*When you ask for the birthday sexy strip, you specifically call it a "boobie tease show" versus stripping, and you buy the slut clothes with this in mind...picking something that keeps her mostly covered up in the other areas of her body but which gives a "peek" at the boobs...or it can be something that can be taken off on top, the rest of her is still covered, but you have also bought her some fun tassles or pasties to be hidden under the sexy top and to be seen only at the end of your "boobie tease show".
See what I'm getting at? Make it all about only what she feels comfortable with. Most women have at least one part of their body they will come forth with if you encourage it enough.