"Loyalty" is another one of those words that gets tossed around, like "courage," "morals," etc.
The bottom line is people, male or female, make choices based on what will be the best possible outcome for that person.
Once you're faced with any given choice, all talk of loyalty can go out the window. Aside from loyalty to yourself, the concept is part of the male Disney fantasy.
A person who stays faithful to his partner does it because that's the best option for him or her.
The incentives may have changed over the decades, but the song remains the same. You might believe that people in the past were more loyal, where in reality they were presented with different consequences.
Even a dog might be perceived as loyal, but he's making a choice not to bite the hand that feeds him.