Oh my, my blowpipe's empty ... where's my stash before I get my sanity back, hurry! Darn, too late ...
Dazed ... in all honesty, you give us readers very little to anchor on, to give you a serious response. Things I think we should know, like how long married, do you have children in the home, etc?
Personally, if you have issues with it, don't do it, and he should respect your decision. You're the one taking ALL the risks ... injury, marital relationship, broken family, disease, etc, etc. Your husband has no "skin" in the game of his own. Men tend to get bored with routine sex faster than women, and the porn isn't helping him any. Looking at too much porn, however, tends to also desensitize an individual's sexual stimulators. Eventually, a person needs more and more stimulation to reach those "highs". It can take a person down some pretty dark alleys, if you understand what I'm saying. Then normal, monogamous sex seems a bit boring to say the least. And, there are many other ways to stimulate a bored lover without submitting your body to someone else.
That said, there's nothing wrong with inviting another person or couple to "play" actually, as long as you do it together and openly. A nice 3-some or couple swap can stimulate a marriage for weeks or months after the event. But, if your moral fibers say NO to anything involving you, then your decision should be final. Let him jerk off to porn ...