I know how you feel.
For me it's not the actual masterbating that's the problem, for me, he can masterbate all he wants but not to naked women doing god knows what.
Yes everyone/most people masterbate but in a women's case( I don't speak for all women) but I wouldn't do it watching porn, looking at other people's naked bodies by myself just to get off.
For me sex is a personal thing which is between 2 people who love each other and see each other naked and don't need anything else to get off on. I don't want to look at anyone else's body just for the sake of an orgasm, I have my husband why can't that be enough.
For me the way it feels like cheating is because I wouldn't go looking online for a naked man to masterbate to, my husband is enough for me.
So it feels like cheating because he has seen other women in intimate ways which I think he should only be seeing me in them ways as I'm his wife.
Yes you can look at someone and think they are attractive and think there sexy etc but to go as far as seeing someone naked getting shagged left right and centre it just makes me feel a bit yuk.
My husband said he was doing it to just get tips for us. So I thought then great our sex life is shite to him but I'm as adventurous as they come so that isn't the problem and if that's the case why not just read it in a book etc rather than get up close and personal with porn hub or whatever sites he was on.
I felt like shit because it felt like he would rather watch porn than have sex with me, I'm here in the flesh when he wants me.
Plus it didn't help that I had not long had my 1st child so felt quite low. We talked about it and I said we could even watch it together but he refused and said he wouldn't watch it again.
What pissed me off is I found out that he was watching it every time he got in from work and was suppose to be having a shower. Then there's me thinking he was just to tired for sex with me after a long day at work but no he already had his jollies in the shower.
I still can't get past it and I probably do need counciling for it as when we do the deed it gets in my head that he doesn't enjoy it or I'm rubbish then I think about the women who are slim and basically everything I'm not or not doing to him what the women are doing in the videos he has watched. It's got to the point where I struggle to orgasm ( sorry if TMI)
I'm done with it now and we don't have sex half as much as we used to. I suppose I'm still angry with him over it. If he wants to watch it then he can go right ahead because it feels he doesn't want me and only uses me to get his release. He can use his hand from now on I'm past caring.
My lady parts are not thanking me though 
At the end of the day even if we can't get past it most people think it is normal for men to do and it doesn't bother them.
Our relationship is good still and we love each other to pieces and I couldn't imagine being without him after all marriage or being in a relationship shouldn't just be about sex sex sex but for most people it is and even if we were to separate seen as its classed as being the norm everyman we meet or may fall in love with will watch porn so we're just stuck or pretend to not give a fuck
Maybe I'm just selfish to think this way.
Have a talk with your partner and really tell him how you feel don't leave anything out.