A marriage is a partnership of respect and equality. Yes, we all have our hang ups and quirks. But, when something as significant as sexuality becomes involved it must be handled in a very delicate manner.
Exhibitionism, which is the desire is be aroused by others seeing, or the possibility (even if remote) of someone seeing nudity/sex, etc is considered a sexual fetish.
In certain circumstances, such as true aloneness with two people who are willing to do so...perhaps this could be exciting or sexually arousing. But, it is usually the result of feelings that are linked to the person's perception of sex...that it is risky or dangerous or needs to be "wrong" or "dirty" in order to feel excitement.
In truth, the excitement about sex is that it is getting to know someone else at an intimate level, not risking one's self or wife to be seen in a compromising position. (This may also be a psychological removal of power for him, for men are to cherish and protect their wives, and here the opposite is being seen, where someone else could lust over your body without his control. This in turn making him feel less powerful and helpless, which may go back to how he feels about himself and his self value.)
In truth, I suspect his struggle with porn and this action is related, most likely from a developmental past where he felt powerless. And, now that feeling of the past is being relived in exhibitionistic behaviors.
In all truth, the fact that you feel uncomfortable is the key. You are not saying that you dislike sex or sexuality, just that you feel uncomfortable in doing certain things. That is not abnormal or unusual. Rather, his pressure on you to do so, is.
Do not allow guilt to motivate you to do something you do not desire to do. There are just some things that couples cannot agree to do together sexually. And, you need to feel loved and appreciated in sex, not displayed.
My take: There are just some lines that couples cannot cross together. If he wants to run around in the buff, so be it. But, he may have to settle for you being a spectator. And, if he refuses to understand this limit, that is his issue, not yours. Steven