Sorry for the rambling that is ahead of you, I am still trying to mull out some ideas and would appreciate any input.
Nature and natural human urges are what people use to explain a lot of things. For example MRAs have used it to explain the double standard where women are valued for their mongamy and men are valued for their polygamy. Known today as not sleeping around vs sleeping around. This is absolute bullshit. Their logic seems sound until you think about it a little more. How primitive cultures functioned was not based in the family system we have today. Today it is an imperative of the male to know if the offspring is his because he goes to work and brings home the resources needed to keep the child alive. He consequently wants to know he is working to further his own seed.
However this is just not how previous human cultures worked. When the menfolk went out to bring down an elephant they would be providing meat for their entire tribe not just their nuclear family, because this family structure simply didn't exist in the way it does today. So they didn't have much concern for what children were theirs and what children weren't because it didn't involve them having to put in any further labour like it does today. This is not nature, this is how our society functions
This brings me to my second point. If you can look at the past and present of human societies and see various "natural" imperatives driving the function of these differing groups then I think the only thing you can say that is natural about human behaviour is that it adapts. For instance in this context you can say it is natural for men to want to know its their offspring in this society, because that is what is beneficial for them, however this too has its flaws.
Which brings me to my third point. One must recognise that humans are and have always been social creatures. We are driven by a desire to improve our society, whatever that society may be, not necessarily our individual lives. When speaking of "nature" you are reducing humans to mere animals (non human animals) and ignoring our cultural imperatives and our higher level of consciousness. What is natural and helpful for an animal isn't necessarily natural and helpful for us. In the example of the promiscuity double standard it can in fact be seen as unnatural for a man to care if his off spring is biologically his because that isn't beneficial to society and may in fact be harmful.
I suppose what I am trying to say is anyone can make up any bullshit and say "nature did it". This is no better then religious nuts saying "god did it".
Human nature-we change
Thats about it.