What has your atheism to do with it, other than possibly being in opposition to religion? It’s not something uniquely “atheistic” about infidelity, other than the lack of dogma.
This nonsense Christian morality that it is so wrong having sex with others during marriage is nonsense.
Not only with christians. The view on infidelity is there in many other religions and cultures other than the judeo-christian ones, and in ancient cultures. For example, in middle assyrian law (between 1450 and 1250 BCE) there were potentially severe punishment for infidelity - “9. If a man have relations with the wife of a man at her wish, there is no penalty for that man. The man shall lay upon the woman, his wife, the penalty he wishes.” Although Middle Assyrian law was particularly draconic when it came to restricting the lives of women, there is good reason to believe other ancient cultures had similar laws. Thus, you’re singling out one target out of many, ignoring that similar law/morality is found in practically all cultures that I know of.
That being said, if we restrict ourselves to current times, the frowning upon infidelity can be explained rationally. When married, the parts have entered a legal contract to support each other, and invest in their common household with money, effort, work, time, as well as emotionally. Thus, having sex with someone else can be viewed as the breach of a contract. Thus, infidelity breaks the terms of the contract, and thus there are grounds for terminating the contract (divorce), unless the parts have agreed otherwise.
Also, I doubt that religious rules about infidelity were pulled out of some priest’s arse and then implemented in the religion, just because he felt like pranking his subjects. Rather, those were most probably pre-existing rules in the ancient societies that were eventually written down as religious laws. This is not a chicken-and-egg question. Most probably the views on infidelity came long before organized religion formulated written rules about it.
My wife during our marriage for examples has had sexual relationships with her ex-husband and I honestly feel no jealousy and am fine with that.
Whatever floats your boat. Others might view such cases differently, independently of religion.
As an atheist I don’t see anything wrong with infidelity in marriages or more than one lover. Christians label it as so evil…but I don’t.
I cannot see what atheism has to do with this. People are different, and react differently to the subject at hand (infidelity).