ok well obviously I'm not speaking on behalf of your husband just gona state my opinion:
1- unless you married a guy with no standards at all! then i highly doubt he is imagining himself with the actress! now that being said i think the majority of guys would imagine themselves in the place of the male actor but with who? that depends on the guy many guys would imagine their wife or girlfriend or crush but i never met a guy who would fantasize about a porn star even single guys would fantasize about a crush or a celebrity or X sometimes the guy wouldn't even know who hes fantasizing about and just think about the actions rather than who's doing them! I'm not saying he would fantasize about a stranger I'm saying some guys would just focus on the feeling and not pay attention to the detail of who the female is could be a wife girlfriend stranger whatever he doesn't even know! its just male psychology, guys get jealous and territorial easy so if he imagines himself with the actress and then sees another guy with her "the actor" the guy fantasizing may feel slightly territorial which would make him feel uncomfortable which would defeat the purpose of masturbation to begin with. so porn is really more of an aid so the guy doesn't need to think what him and whoever he's fantasizing about will do next he just follows what he sees and applies that to his fantasy. so its more about the content rather than the actors or actresses. kind of a prefabricated fantasy that takes out the effort of guessing
2- im sorry if i offend you, obviously your entitled to your opinion and you determine your own limitations, but I'm just curious, how is it cheating? the viewer most likely doesn't know the girl or ever came in any contact with her she most likely doesn't know the viewer exists so its all imaginary! i mean i could definitely understand how it may hurt you if your husband fantasizes about her since you are his wife and in a perfect world a husband would only fantasize about his wife but as i said if your worried about the actress being in his fantasy without even knowing anything about your husband I'm almost certain he doesn't fantasize about her, i'm not saying you are definitely who he is thinking about that depends on him and who he is but at least its not some disgusting whore who's profession is sex with strangers! might not make it better but maybe not as horrible.
3-now as for him saying sex with you is stressful if you think about it from a totally biased position you could kind of see his point... meaning sex with another person maybe seen as half pleasure half duty, but i think its a selfish way of looking at it and he definitely should never say that to you or act in a way that indicates that he's thinking that way i mean if he's a single 20 year old having a one night stand its still selfish but not all that big of a deal, but a man should feel pleasure when he pleasures his wife or girlfriend since he cares for the person and wants them to feel good so maybe if while talking with him about that topic you show him that pleasuring you isn't necessarily a chore but actually another form of pleasure, maybe not physical pleasure but still pleasure! idk its a matter of how a guy looks at it so present the idea to him in a different light and he might see it differently
4- as for you feeling upset or frustrated when you aren't done and he doesn't want to continue, no one can blame you for that cuz most likely "i'm speaking generally not you specifically" you reached a higher state of arousal than you were in before you started because you were pleasing him so he should at least make an effort if he's finished before
idk my honest opinion is a guy who watches porn isnt nearly as bad as a guy who has a girlfriend on the side or meets up with prostitutes in some motel and im sure you know that there are far too many husbands who do that
im not saying you should like he watches porn but at least accept it as the lesser evil
or you could even try to make it work towards your advantage and maybe even watch it with him, im not suggesting you fantasize about it but maybe just to get some ideas to try with him you may enjoy the different and new experiences and i think he would be even more excited to have sex cuz fantasizing about it may be pleasurable but the real thing would be better, u said your sex life is good but i'm sure there is always room for improvement.
but in general i think you are doing more harm than good when you argue or fight over what or who he fantasizes about, thats all in his head and its simply imaginary nothing more and you never know he might really be thinking of you i mean give the guy some credit he did choose you to spend the rest of his life with!
it just sounds to me like worst case scenario hes a little selfish in bed rather than a cheater
anyway good luck to the both of you and i hope you two get this all figured out and im sorry if i offended you in anyway because i definitely didnt mean to do so I'm just stating my opinion