I have to disagree with you. He is asking why men evolved with this fancy, or rather why a high percentage did. They're are some heterosexual men who do not but a lot do. I myself have questioned the scientific reasons behind it and I think there are multiple reasons.
Bisexuality in women makes it easier to nuture both sexes.
Men instinctively wish to spread their genes to as many women as possible. If your wife also has a wife then you have 2 wives as in a harem.
Most men lack the desire to always satisfy a woman rather then get off whereas bisexual women prefer to satisfy eachother.
Many women find eachother very attractive visually like men. In fact many of them find the same features that show reproductive health attractive and are more attracted to men romantically but to women sexually which creates lubrication of the vagina for both women which is desirable for the male counterpart. A biological reason for this would be a dominant male would have many wives and it would be in eachother's best interest for all the women to be healthy for many reasons like infecting the children. Remember women evolved with an instinct to nurture that was historically their role during the course of evolution. This is why even today women are ins more likely to wish to engage in threesomes with a man, or even just fantisize about it if he has money. And so I would imagine it's much more likeky that a rich man can have multiple girlfriends if he wants then a poor one past a certain age. I've heard stories that rich men have private mistresses that tbey keep secret. A cultural/evolutionary reasoning may be rich men subconsciously realize that all men can't afford a lot of women so it's better public relations to set the example for all. It's an argument like some men feel you should hide porn, while others wish to shoot it.
No I know what you're saying the first porn I ever saw was a man and woman nude and fantazing about the woman's butt aroused me even though the man was there and seeing her enjoy a head in her butt, and I've seen porn where I get turned on by this girl loves to give oral sex, or looking at how she looks but I prefer lesbian because it's more women the idea of having a lot is a turn on, and I don't have to tune out the fact this guy is having sex with her and I'm not. I don't know like I find the idea of doing things with a girl hotter if the dude isn't there. I'd say I probably like girl girl, solo, and heterosexual in that order. Not that I think dudes that only like to watch that are gay really. I would watch it if the others didn't exist and that was all they put out. Just like I would eat a hamburger if I didn't have a cheesburger but prefer a bacon cheeseburger with mayo and onions, maybe an onion ring.
I agree this is a biological issue a a lot of religious nuts who are unwilling to accept this are annoying to me. Maybe they can just become priests and worry about other men's sexual likes all day.
I do agree that most women are more tuned on by other women sexually but not romantically as it has excited every girl I've ever had sex with and they were of various types and races. You also can't find one one female orn actor who has never done a gg scene but you can find aot who will only do gg but aren't true lesbians in that they'll have sex with a lot of women but only one man. I would imagine monarchs had similar situations.