I can explain this easily as a guy. While we often enjoy watching porn, even if its not seen as an act, guys would often react as your boyfriend is...but why?

Let's look at the specifics of things. A guy viewing porn is inserting himself into the situation observed. The person involved in the porn itself is not necessarily the focus of the fantasy internally. This is a one way street however. While your man viewing a porn isn't necessarily equating the woman in the porn to an actual person, but rather a sexual icon that is unrelated to the actress as a person; he cannot seperate the idea that others viewing you on tape would view you as the person and not as he actually does with the woman in porn he watches. The problem is now, in his mind, anyone watching your video would now be a "threat" to your relationship, because they have an attraction to his girlfriend (albeit this is a ridiculous notion, often times emotions and jealousy have ridiculous roots). Think of it in other terms. If you and your boyfriend were out in public, a club, a restaurant, wherever. And some random man walked up and commented on how sexy you looked. Do you think your boyfriend would take this as a compliment to his resources in having acquired some a beautiful girlfriend as to have random strangers comment on it? The man who would not mind making a porno for mass distribution with his girlfriend, is the same man who would appreciate such comments from others about their girlfriend. It all falls into his self-confidence in relationships as well as threat perception from other possible suitors to you (whether te risk exists or otherwise)

So to answer you question, is it a double standard? No, not exactly, because his viewing of pornography does not touch on any sort of emotional playing field and is purely a substitute for fantasy, unlike women who typically can use their mind as the origination of fantasy material, guys need the stimuli.

As for another man being the partner; I guarantee that would be the downward spiral of your relationship were that to happen. And were he to play the male role, as I stated in the former paragraph, its a proxy jealousy of another man having any interest in his girlfriend.