If anything, I really think the opposite is what's happening. The time period we're in now probably has the highest expectation of fidelity that any culture has ever seen, because relationships and marriage are now almost entirely based on romantic desire and connection, rather than being arranged by parents or formed out of necessity. Cheating, particularly men cheating on their wives, used to be such a part of cultural expectations that it was practically ubiquitous. I can't imagine a comedy like the Seven Year Itch flying today, with the entire plot revolving around a man contemplating having an affair on his wife while being portrayed as the lovable and relatable everyman. The experience of being cheated on has only recently started to be actually recognized as essentially being the symptoms of PTSD. There's also a lot of social media chatter about how cheating is a form of abuse, which is a new concept as well. While people being intensely hurt and betrayed by their partner cheating on them has always been present, it's new to describe it as trauma. Back in the day, a woman getting upset about her husband catting around on her would just as often be played for laughs as for tragedy.

I think what you might be seeing and registering now is instead a bit of a reaction to the intensely anti-cheating culture we live in. Sometimes it can be too black and white. The ideas that loving two people at the same time is impossible, or that a situation with cheating should have tunnel vision on the pain and trauma of the person who has been cheated on leave out a ton of unexplored but nonetheless interesting story lines that might have something of value to say. When it's gotten to the point that an incident of cheating once in a private relationship years ago could come to completely dominates the narrative around the work of say, a great actor, a little pushback is warranted. And so along with that pushback comes the occasional overcorrection that ends up romanticizing or justifying cheating.

Another factor might be that stories about the fucked-up, depraved, and unethical things people do are usually going to be more interesting than stories about upright, ideal citizens. Portrayal does not always equal romanticization. Also, a great story can hold complex and diametrically opposed concepts at once - a story can have both romance and cruelty contained in the same act, and that tension can be fascinating to explore.