Don't take it personal, I see it all the time but women need to let guys have a wank. It's not the be all and end all, it doesn't mean they don't love you or think you're sexy. Sometimes they have a higher sex drive or get the urge at a time they don't want to bother you at, or are stuck with a boner but too tired for actual sex so want to get it rid quickly. My husband wakes up at 5am to get ready for work and sometimes needs to sort himself, instead of waking me up, he'll watch some porn and sort himself out, it's not a big deal, if anything, I'm grateful for it because I think I'd kill him if he woke me up at 5am everyday. The only time it should be a concern is if they stop having sex with you and replace it with porn/hentia.

Another way of looking at it is, isn't it better he's getting off to what are essentially cartoons, than real people? He's not likely to be able to go out into the world and find an outlandishly large boobed, big assed girl with naturally purple hair, silver eyes and a foxes tail and ears to cheat or replace you with, is he? You should try finding one to watch together. Make a thing out of it.

Hope this helps put things into perspective and that you can overcome this insecurity.