I agree with other commenters that he probably can't handle the truth. A lot of people think the idea of a promiscuous partner is hot but then insecurities will start to surface. That being said there is a small portion of the population that actually would be aroused. It will be up to you to try and figure out which type he is.
For your part, what would it be like to live with a lie even if it's to keep your partner happy? I have had relationships where I kept what I wanted in the vanilla category for my partner and none of those relationships have lasted (one went on for almost 10 years).
Being open and true to yourself is one of the most liberating feelings ever. You need to balance that with the current and future state of your relationship.
