Its not "normal" to watch videos of other people fucking it's been "normalised" by quite alot of people. I personally see it as cheating as what's the difference between your partner receiving a sexy strip tease from someone in real life or lusting after somone on film.
Tell him clearly you're not OK with it and if he respects you he'll stop."
Bloody hell who invited the Puritans back.
Ultimately unless you consider porn to be against the 'fundemental rules of the relationship', in which case the only option is to break up then you have no option to like it or lump it. The only other alternative would to be seriously controlling over your partner, in which case id be telling them to run.
Pornography and mastubation can be traced back to the ancient cultures. The vast majority of men (and I suspect a signifcant minority of women) engage in it.
As for 99% of it been violent and extreme porn what utter cr@p. The vast majority of it is fairly vanilla hardcore. Sure you can find the extreme stuff but then you have no idea whether the women are consensual or not...might suprise some women but some other women actually enjoy things like rough sex, BDSM, dominance etc. Assuming no women in porn consent is a digustingly narcassistic 'saviour syndrome' from middle class women who should concentrate on people asking for genunine help not arbitrarily painting women they have never met as victims against their will just to satisfy their political ideology.
Are some women abused and exploited by the porn industry? Of course...but so are some teen actors by the film industry...is anyone militantly opposed to watching the results of that like Home Alone? We enjoy many things that may or may not come from exploited people from our clothes to our TVs. We rely on goodwill from the suppliers that it is not but very very few individuals extensively check.
As for OP.
Men on average are doped up on testosterone. The side effect been significantly higher libidos than women (on average). Truthfully you could ride him all day and it wouldnt satisfy it completely.
In many cases chosing to mastubate over sex is usually a convenience thing (quicker, less fuss etc) especially if they are busy on the go people. Aslong as you are having regular sex which you are happy with and porn isnt a dealbreaker (as much as I think for it to be dealbreak is absurd) then I wouldnt overly worry about it.