Apart from all the hurt this is clearly causing to some, what about the money spent?
As someone who relied on her ex to support her for a while (job loss), I would be fuming if someone who grudged paying for a meal out for us went and blew money like that on "other" women!
Having said that, I never did work out how much he spent on porno mags (way before the internet, here). I think I was too shocked at finding three 4 foot high piles in "his" shed! At a fiver a throw he must have spent a fortune.
I get upset reading how upset this causes others to get. I really wish the weaker, easily led blokes would man up and tell their friends to sod off. I worked in a male-oriented industry for many years and saw the same "macho" blokes pushing others, and others giving in just not to look "under the thumb" (apparently THE worst thing that could happen to a bloke according to macho types!).
It all boils down to having respect for the person you love. Is your bloke clueless, a wimp or just plain does not care enough? I ended up having zero respect for my ex (and that was before the "find") and would actively look to do things that would piss him off. The only thing I learned from this behaviour was how true cutting your nose off to spite you face is. Oh, and that I was, and still am, relieved to be shot of him.
I'd hate to say to others, yes, dump him, and for it to be the wrong thing. But for those who really can't live with this behaviour, you have to make him see exactly what it is he's done to you. You cannot let anyone get away with making you feel like this when they do not understand or trivialise why.
Don't let anyone, partner or people here, make you feel like your feelings are irrelevant. It took me years of therapy to realise that my feelings are mine and just as valid as anyone else's.
Good luck to anyone struggling with a partner who has hurt them and are at a crossroads.
PS Sorry if this is rambly - I know it's never a good idea to post after a glass of wine. I just couldn't stop myself!