Yeah, but let's not pretend that this is a thing everywhere now either.
What do you do when your SO has been unwilling to have sex for an excessively long period of time? (Go take a look
here for example). Would they still have the right to complain about it if you end up being intimate with someone else?
What if the SO is a little bit childish and wouldn't accept any kind of 'adult conversation' about the subject? And let's not pretend this is rare now either, there's a lot of ... not too amazing / great people out there that cause major fights over petty things or just can't be reasonably engaged for an honest talk.
At that point I wouldn't blame a guy or woman for saying:
"I'm gonna be fucking another person. Deal with it or piss off."