The best way to approach the question is to open up a discussion about his 'gross' comment. The ideal is to address his words as close to his uttering them as possible, so that the thoughts and feelings that prompted the comment are still fresh in his mind, and therefore available for exploration.
You can say,'I wonder if you have a few minutes for a heart to heart talk right now.' When he gives you the 'green light,' then tell him that you noticed that whenever he sees a scantly clad women with too much cleavage he says 'gross' and you were wondering what he was feeling when he made the comment.
If he is open to talking, you might go deeper and ask him how he feels about nudity and sexual activity, in general, and how he feels about the prospect of being intimate with you. You could also be direct and tell him that you are worried that he has mixed feelings about sexual involvement and that you are concerned that your sex life will suffer.
Once the issue is on the table and you see how he responds, you will be in a better position to judge whether he is or isn't sexually inhibited. If he admits to having issues, then you would be wise to encourage him to talk to a therapist right away. Good luck.