If you look at the replies you have here then we can see that everyone has an opinion ..... and mostly they do not match.
Now I could tell you how my PhD Dr wife and myself have been married blissfully for the past 30 odd years, how we form an egalitarian, symbiotic Gestalt unit ... I could tell you how that all works but it is pointless ... because you are not us.
What I can tell you is this and this is 100% irrefutable fact, not an opinion - The more like one's partner one is, the more likely your partnership will be long, successful and happy.
Yes all sorts get together; but then look at the divorce rates and those who lie in endless angst and misery.
In essence the closer a couple is in terms of origins, socio economic group, belief system (or none), political views, views on family, friends, socialising, importance of work, money, sexual interests and so forth .... the better.
My wife and I are essentially an exact match
, the real difference we have is our thinking style; mine is binary and planned, but I am far from risk averse. Whilst my wife is definitely mad professor, she has an 'image streamer' mind, she sees patterns where few others do. Therefore together we are Gestalt.
It is obvious you know the answer to your own question but just do not want to accept/admit it .... Yes of course it is unacceptable and if he is doing it on the internet then he is doing it in real life ... There is an old adage that goes "If they will cheat on someone to be with you, they will definitely cheat on you".
Your boyfriend is all about himself and his needs .... Not much of a catch is he; throw it back and catch a much better fish.