That’s a moral statement, and as I stated at the very beginning of my post, my point relates to what’s going on aside from morality.
That’s probably why you don’t think what I said made any sense, when in fact it’s where contemporary objective behavioural science/ethology is at - just ask Robert Sapolsky amongst others. It’s not their personal opinion, morality nor wish - nor is it mine.
I’m not saying that males should view females as sexual objects, nor even that they do, I’m just saying that with the sexual dymorphism of mammals such as humans, and due to the different roles of each sex in the reproductive process being as they are, sexual selection necessarily tends towards what I was explaining. For better or worse.
Obviously for a female, the male tendency (n.b. not the rule) is worse. Since the cost of impregnation for the female is far more, the more they get back from others after incurring such costs the better. For them, it’s far better for the man to see impregnation as only the beginning of a lifetime commitment - to want fatherhood and for it to involve deep love, heartache, hard work, sacrifice and quite a bit of money - as you say. But for the male I’m afraid this simply isn’t necessarily the case.
Sure it’s better for both parents to invest in their kid to give them the best chance of carrying it all on - those who want/do this are going to be the ones that emerge more numerous. But a male with offspring by only 1 woman won’t necessarily be getting the best value for his efforts if he puts all his eggs in one basket (so to speak). Due to diminishing returns, min-maxing for the most desirable males means they can spread their efforts - preferably with more than 1 woman in case the one they would have otherwise remained faithful to wasn’t as desirable as another female with whom could also invest their time, money and energy. They can’t spread themselves too thinly, obviously, but in some cases it can even pay off to impregnate and leave the female for another altogether - rolling the dice that the abandoned offspring will be desirable enough in spite of their shitty hand that they were dealt. Also, the then single mother can potentially win over another male to invest in the child - perhaps even starting their own family with each other and adopting the fatherless child into it.
Also, it makes sense for the male to only invest for a limited amount of time. It’s no coincidence that relationships seem to have a time limit, after which both parties want to move on. Sure the time limit can be denied through force of will etc. - easier if you were brought up in a time when social norms were to force this. In some cases there might be no time limit, or a longer one at least so it’s genuinely preferable to stay together for the long long term, but this is by no means the rule. However, there are advantages to both grandparents being around to help with their offspring’s offspring - so there is at least some demand for it.
For the female, all this lifelong monogamy and loyalty is highly desirable: the returns don’t diminish as quickly as they do for the male, but the female situation isn’t the only side of the equation. The societal value of monogamy can be an indication of how much power females have, although it can be that the males are benefiting in other ways as a trade-off. It might even be a farce that is undermined only secretly - and there might be a denial but an underlying knowledge by everyone that it happens, and that one ought to tolerate but allow it implicitly.
In short, it’s a complex game with many stable solutions that may not even be constant over time. To moralise over it seems to be ignoring of the nuance inherent in the system.