Hello! I have been with my partner for 10 years, we have 2 beautiful children together and generally have a very easy-going and loving relationship. The one thing I am not okay with is porn. To me, it is fantasizing about being with another, probably better looking woman etc. (By the way, do men do this?! Like see a woman in the street and imagine having sex with her? The things I find on forums are surprising for me

) Okay sure, maybe on a rare occasion it is whatever - it would not bother me much, but the thought of it being regular is almost sickening for me. He knows I don't like it and denies watching it, I have looked in his phone a couple of times - which was always going to be fruitless given incognito mode anyway. No part of me ever wants to read messages, know his location etc. but I would 110% want to know if he watching porn and what it was. Nudity in films etc. is absolutely fine, but watching somebody have sex with a half-silicone woman and getting off to it really bothers me.
I searched this issue I have to find that it seemed uncommon... most comments addressing this were brutal and saying the concerned partner was jealous and insecure and needed therapy and their marriage was doomed... is this true? Is it so bad to be jealous?
He is mostly an honest person, unless it comes to something he knows I will be upset about - like spending too much money on something stupid etc. in that case, I have found him to withhold information from me before. Or at least play it down massively.
He was my first boyfriend and (only ever) sexual partner as an 18yo, and we were friends before that. In the time I knew him, I knew him to have several relationships and typical teenage boy habits of trying to sleep with as many girls as possible! I was very different - I partied but saved myself for somebody I could connect with. I think the insecurity stems partially from my inexperience with anyone else, and partially from how hard I am on my self-image.
Is this really such a problem, or a sign of issues within a relationship that need to be addressed?