I'm not much for psycho analysis but that sounds like a fetish of his. Especially if the porn is consistently the same subject matter. Have you offerred to let him watch you masturbate?
If he's strongly opposed it it. Don't be too upset. one of the most common reasons men in relationships continue to watch porn is because of a dichotomy between fantasy and reality. Most men need the stability of a good woman in their relationship and feel a strong desire to treat her as a thing to be protected and kept pure. However conversely they also have a desire for a woman who is filthilly debased and hyper-sexual. Not unlike the Madonna/prostitute complex. For a lot of these men, if their significant other behaved like the women he drewls over in porn it would horrify him.
Offer to be his porn star (In real life, no filming) if he freaks out about his wife masterbating in front of him, tell him he needs to get over it, you are not willing to compete against his sexual fantasies for his attention. Even if it only means being close with him when he watches porn and sharing that moment with him you're still not being locked out of that part of his sex life.