Tzniut (modesty) can be expressed in every aspect of our lives. So tzniut can, and should, find expression within marital intimacy. But it will manifest itself there very differently than it does in public. As such, it is permitted to wear a provocative nightgown to bed. It may even be encouraged if it can have a positive effect on your relationship. At the same time, you are not required to do so if it makes you uncomfortable.
We think the first step is to clarify to yourself what about it makes you feel uncomfortable. Would occasionally wearing a more provocative nightgown make you feel like an object? Why (not)? Does any part of you find the idea appealing? Is your discomfort a matter of clothing or is it related to attitudes about intimacy in general?
To understand your husband's perspective, perhaps you could ask him about it. Why is he making the request? How does he feel about it?
We think there are many possible courses of action. The best course of action is one that builds on both self understanding and an appreciation of your husband's point of view.
Please read the article on intimacy by Rabbi Avraham Peretz Friedman, which appears on our website, and don't hesitate to be in further contact with us.