Isn't it quite standard that people in general look out for their own interests and happiness?
Yes, and that's not necessarily a bad thing.
I feel like some men try to convince themselves that men are inherently more principled than women, believing that females are the only gender with the exclusive capability to act in self-interested manner. As if believing in such a proposition gives some sort of comfort and rationale for a ego once bruised by a woman.
Women, generally speaking, will do what's best for them. I have no problem with that, because I too, will do what's best for me. Most people, as you said, will generally do what's best for them (or at least look after their own interests). Can that sometimes negatively affect someone else's life? Of course. But I don't understand why would pretend that only women are capable of that?
To OP:
Women are manipulators? Lol because we men have
never tried to influence a woman according to our
own self-interest? Even when our efforts to get what we want are carried out benignly for the most part, we are still placing our own interests first. There is nothing wrong with that. I guess it depends on how you define manipulation, but I would reckon most people engage in some form of manipulation to some degree every day with the intention of looking after what their interests are at that moment. It's a human trait, not a gender trait.
Women's priorities change and their personalities are situation? Again, this is not exclusive to women. If you started dating a girl who gave you great sex, great companionship, and stimulating conversations for the first 3 months, and then you started to perceive (whether it was true or not) that the sex started suddenly getting vanilla, her company was no longer enjoyable, and she was no longer fun to talk to, would you act differently towards her at the end of the 3 months as you did at the start? Probably. Would your priorities regarding her in your life change? Probably. You would be acting according to the situation, and according to whether your own interests are looked after. What if in that situation the sex, companionship, and conversation was objectively the same, but the novelty just wore off? You'd lose interest, your priorities would change, and you will act accordingly. I agree that it sucks for guy or girl who has to deal with consequences of that, but men and women aren't horrible for acting in their own interests.
Obviously men and women have different innate interests and priorities, I am not disputing that, and I agree that we need to be cautious in our interactions. But that doesn't mean us men ourselves do not act self-interested and that we do not change how we behave based on the circumstances. It's a human thing, not a gender thing.
Yes, there are MANY differences between the genders, but self-interest is not one of them. I see no utility in pretending it is, besides providing some men with some sort of comfort and rationalization for their life situation and experiences.