Jumping in here from the academic side (I teach history at the post-secondary level, just to flash the credentials), that's utter twaddle. (that's the technical term.
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The entire concept of marrying solely for love is an extraordinarily modern concept (as in, the past hundred years - it's modern to historians!). Not even touching non-western societal models in this, the notion of marrying and being sexually faithful to one person, regardless of marriage vows, was pretty much only ever applied to women, as a means of maintaining control over female sexuality and - more importantly - purity of bloodlines to establish paternity and patrilineal inheritance.
In the 16th century in England, Robert Dudley's marriage to Amye Robsart was decried publicly as "a carnal match" and thought to be most improper, because they were known to (shock!) be in love with each other! Before they married! How rude! (
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In the 17th and 18th centuries, the cult of 'la maitresse' demanded that every man who could afford one, should keep a mistress. Or more than one. There was an official position in the hosueholds of the aristocracy for the "maîtresse en titre" - the man of the house's official mistress. (As opposed to his wife, whose role was heir-bearing, or his other side pieces who were there for fun.) As an unmarried mistress, the maîtresse en titre played political roles, acting as hostess for the gentleman's friends and their ladies.
Resource: http://www.18thcenturycommon.org/350-years-of-dangerous-women/
In the 19th century, the Victorians particularly became obsessed with sexual morality in a way that eclipsed all other understanding of marital fidelity, and affairs went underground. But for men, they still continued pretty much as they always had been. The women are the ones who lost out, honestly, because a mistress' status plummeted, from the possibility of achieving social power through her link with a powerful man, to being a kept woman, and a dirty little secret.
Resource: http://www.stephaniecoontz.com/books/marriage/chapter1.htm
In Europe, during the twelfth and thirteenth centuries, adultery became idealized as the highest form of love among the aristocracy. According to the Countess of Champagne, it was impossible for true love to "exert its powers between two people who are married to each other."
It's only in the twentieth century, when marrying for love took predominance over marrying for economic alliance, that we began to redefine how seriously (men) we were meant to take marriage vows that included fidelity.
So lord, by all means, feel betrayed, feel hurt, be devastated by the breach of promises made to us by the people before whom we are the most vulnerable. I'm not saying any of this to suggest that cheating is in any way excusable in the modern day! But to say that we've somehow become 'more immoral' in the modern day (subtext: because of women's sexual liberation and the changing dynamics in the family and the workplace) is absolutely and empirically untrue.
(And for the record, there were lots of workplaces in the early modern era and age of enlightenment when men and women worked together. Women didn't get forced out of the public sphere in the west until the 18th century.)