Humankind has evolved to the point where we can clearly see--if we choose to do so--the short- and long-term consequences of "over-breeding". Animals that over-breed don't have enough to eat, and they die during famines--or they'll die yearly, during those times when food is seasonally scarce. Or they're weakened in their hungry state, and are more-subject to predation. For non-human animals, life is about having enough of a species survive every year to carry on that species. Our smart brains have allowed us to realize that we are simultaneously animals and beyond-animals, both a part of nature and a force of nature. Unlike animals, we have Free Will.

Humans tend to care about themselves, as individuals, first, their immediate families second, and their communities, their national/religious/ethnic affiliations, and their entire species in descending order. Therefore a given woman's desire to become a mother might override her realization that there are "too many children" to be supported in the area where she lives: She'll exercise her Free Will and go ahead and breed, anyway, and hope that HER child gets enough to eat to survive to adulthood. In a different scenario, another woman--an atypical woman who lacks any maternal instinct whatsoever and finds the very idea of the motherhood role to be abhorrent--will put her own interests against political/religious leaders in her region of the world who say that a too-low birth rate is hurting her county's economy. If she gets pregnant, then she'll seek out an abortion--contrary to the special interests of her nation--because she's exercising her Free Will. In this case, Free Will has given her the means to do something that would be beyond the capabilities of any other animal, and so could be called "unnatural." A recognition that not passing on ones genes is a "greater good"--not only for oneself, but for everyone--is unique to humans. A lack of maternal instinct is NOT something that should be carried on through breeding--that much seems obvious to me...but not to those who insist that all women have these perfect hormones that "naturally" lead them to embrace motherhood, and that this is the only valid role for every human female.

In a similar vein, one has to wonder why homosexuals continue to exist, given that common sense would dictate that homosexuality discourages breeding. How is it that these "gay genes" continue to be passed on? This is counterintuitive, in terms of evolutionary theory. The answer is that homosexuals ARE breeding, heterosexually. They are doing it 1-because they might desire to have children, 2-because they're not EXCLUSIVELY homosexual, and 3-because they are reacting to the widespread disgust with/hatred of homosexuals that their fellow-humans express--the end result being that they're "in denial" about their homosexuality and make babies before they admit the truth to themselves or they marry and breed to shield themselves from their community's disapproval. Again, Free Will is at work: The individual decides to go against his/her inborn sexual desires and breed heterosexually. (This is one reason why those "Ex-Gay" Christian fellowships are counter-productive to those fellowships' goals: By encouraging gays to breed, they're only ensuring the birth new generations of gays who might not otherwise have been born. Of course, they don't BELIEVE that homosexuality has a large genetic component--they think it's all the work of the Devil.)

Can humans be convinced to put aside their self-centered interests for a "common good"? Yes, they often can be persuaded that Free Will is an evil thing (refer to the Garden of Eden story), and that the only act of Free Will that a human should commit is to decide to voluntarily give up his/her free will and follow the politico-religionist leaders, who all Know What's Best For Everyone Else Because They Have a Direct Line to God's Will and Universal Game-plan.

Humans will do whatever is "natural", because we're a part of nature. Hence, most of us will breed--regardless of whether that breeding would benefit other humans in our area--because our instincts shape our personal desires. Those of us who choose to not breed don't get to pass on our genes, but that doesn't necessarily mean that we're some sort of a drag on the entire species (as the politico-religionists would have us believe). When there are six or seven billion humans on the planet, it's better for the survival of the species to raise quality children, from quality (i.e, ready, willing, and able) parents, than it is to go for a sheer quantity of babies.

As for the supposed "unnaturalness" of mariage: We marry, in large part, for a couple of reasons, the first one being that human females have a harder time with birth than many other species of females, and the females and their young tend to have higher rates of survival if they have a "committed" male caretaker. A second reason is that males figured out that if they can control females, then they can control paternity AND (hopefully!) have a reasonably-certain sexual access to a given woman at any time. As you said, though, the tendency for males is to be promiscuous, and this flies in the face of marriage--at least in those cultures where marriage is defined as mutual fidelity between mates. Many other cultures have marriage systems in which the more-powerful men get to "hoard" all of the breeding females. In that sense, marriage, which you have labeled as being "unnatural" (at least for males), could be defined as being a culturally-imposed expression of males' evolutionary goals (survival of offspring, control of females' breeding access, and ensured access to females).