It’s so culturally ingrained that I’d be amazed if some aspect of this weren’t in a couple’s monogamous relationship, particularly if they had never really talked it through about what they want and expect. Most people just plug in to what the “usual” paradigm is and even in modern times, that involves this patriarchal aspect. We did it too, with no malicious intent, but just because it’s the program running in the background unless it’s actively challenged.