Lets be straigth forward on how would things be with this "Order" :
There are more women than men in the world, but only barely. (not like 10 to one, but like 49 to 51 kinda more women)
In order for men to have sex the way nmartin proposes and women to not to, there would be A LOT OF MEN that would never get laid.
I explain:
We have 20 woman and 5 men. Let´s say each men is alowed to have sex with 4 women in his live and women with 1 men in their life (so we would be saying that this "order" is making men hacve 4 times the amounf of sex partners than women which I am sure is much less than what nmartin suggests, but let´s say it is only 4 times the amount for argument´s sake.). the systems works, because there are a lot more men than women.
Man- Woman woman woman woman
1st Man- Woman woman woman woman
2nd Man- Woman woman woman woman
3rd Man- Woman woman woman woman
4rd Man- Woman woman woman woman
5rd Man- Woman woman woman woman
Now, let´s say the system is actually like today´s world. It would be more or less: 19 men and 20 women. It would look like this:
1st Man- Woman woman woman woman
2nd Man- Woman woman woman woman
3rd Man- Woman woman woman woman
4rd Man- Woman woman woman woman
5rd Man- Woman woman woman woman
14 virgin men that can´t have sex with any women
All of them have already had sex with the amount of men they are allowed to be the "order of nature".
So now this "natural Order" is making 3/4 of the men to be completely unable to have ANY sex partner.
How can they get to have sex? The natural Order has an answer!
Rape
It is proven and indisputed by biologists that our species, as one of the species were the male is capable of raping the female to have sex (this doesn´t happen in all species) is naturaly predisposed to this scenario from a buiological "evolutionary" stand (naturaly civilization evolved awahy from it, as sociologically wise, but we are still bilogically apt and prone). For this species that are biologically cappable, it always happens when the male cant take a female by normal mating.
So, according to the "Order of Nature", men should be raping women more and this shouldn´t be ilegal.
Or do you se any way arond it? cause from where I stand, this whole "order of nature" thing is completely ridiculous. As someone else said, the monogamous relationships aren´t even very natural for our species.
We made that because we found it MORE FAIR and convenient in many cases.
The issue with the sex would be the same.
Or rape.
Personally, I kinda favour simple sexual freedom of the woman to be what it is: perfectly natural and only immoral for those who are blinded to how would this work.