^ninja'd (sorta) :(
Oh my god... OP, you are painfully over-simplifying the complex and multi-pathed map that is human reproductive strategy... then you go further to make yourself appear even more ignorant about the vast diversity of reproductive strategies exhibited by a number of other animals. I really cannot believe it.
Before we talk about anything, let's remember that raising human offspring is totally different from any other organism on the planet, and for one reason: The human brain. The human brain is so huge and complicated, that it causes tremendous complications during reproduction. It takes an extensive time to nourish (9 freakin' months), and is so big it literally endangers the safe birth of the mother. Furthermore, in order to facilitate the birth of such a big brain, evolutionarily speaking, all humans are born prematurely, being completely helpless and blind at birth. It takes a lot of available resources (food) and security (protection from predation and environment) to make this dangerous strategy viable. Fortunately, all of that is provided by human society, the brainchild of the human mind, ensuring we have the food and security needed to raise human offspring--of course, society requires technology and culture to function, so humans have to go through extensive education in order to keep it running.
tl;dr: Human kids take a SHIT load of work to bring up, and that makes human reproduction a very special proposition.
Getting to the actual strategies, I can't believe you can talk about genetic/reproduction strategy without talking about the inherent difference that lies between male and female strategies.
Females have only 1 egg per month, and must carry a child for 9 months before it's born, all the time it is sucking up valuable resources from the mother's body like a parasite, and the birth itself can be dangerous. On the other hand, men produce millions of sperm on a constant basis, and can mate "freely" without risk of pregnancy, and have much better strength and speed with which to face natural hazards. This obviously means their reproduction strategies are going to be radically different.
Obviously human women will have strategies based on security. They need protection and food, for themselves and for their offspring. This is extremely difficult for a lone woman to provide, especially during the months of pregnancy. Obviously, it will be much more beneficial for her to find a mate with which to work with--this is where the benefits of marriage, one-on-one relationships because much more obvious. Without a male to help provide for her and her children, reproduction becomes difficult, and so women are evolved to try to "tame the male." You now know why women are so into making bonds, and are so happy to get married-- doing so vastly improves her chances for reproduction.
On the other hand, there is still the roots of old mammal instincts that demand she obtain the genes of the "best" male, and now you know why many women sleep behind their man's back. It's sad to say, but the built in priorities are (1) finding a male provider and (2) getting the best genes for the offspring. Sometimes, those two goals don't coincide on the same male.
More often they do though, especially when there is a big risk of losing her provider if she cheats, and also it's very difficult in modern society to judge which traits are "more fit" to begin with (since let's face it, strength, speed and sometimes even intelligence are pretty unimportant when it comes to providing in modern society).
By contrast, males could theoretically impregnate countless women in a single month, given the opportunity. This simple truth is still rooted in our ancient mamalion male instincts-- and many men never get past the roaming and romping to "spread the wild oats."
But, marriage wouldn't exist if men had no reason to settle down.
First, the female ovulation cycle is designed to almost force us to do so. Unlike in many other mammals, human females have no "heat," there are no obvious chemical or visual signals that a female is ovulating. Furthermore, there is no mating season--women are fertile all year, but only in the most narrow time frame, and unlike females of many other species, human women's reproductive organs are very unreceptive to actually reproducing. The vagina is filled with sperm-killing acid, and the uterus full of sperm-killing (and sperm-targeting) white blood cells-- the chances of any singulary instance of sex leading to pregnancy is dramatically low compared to many other mammals.
Why guys? Because them ladies want to force you to stay to make sure you got the job done--have a happy honeymoon, and know why human males are interested in sex with the same female repeatedly. Why? Well it comes back to the premise of human reproduction--you need tons of resources to secure a healthy human rearing, and a lone mother often can't get the job done. She's built to force you to stay. You know that without you, her odds of successfully reproducing is very low--that in and of itself is a huge incentivizer to settle, even for males. No matter how many women you sleep with, even successfully impregnating many of them, that's no guarantee that any of your offspring will actually fare well after.
...except for the fact that one can always fall back on the graceful hand of society these days... Social Service kind of fucks hard with all of this, just saying... You know what the best strategy to work the system is? Have lots of kids with different partners and have them all put up for adoption because some hard working folks out there will get cuckoo'd into rearing your genes... Or else the girl you ditched will be saved by government handouts. Sounds good!
...taking a step back from my possibly tasteless rant, let's mix society into the mix. Remember that society is the big insurance for humanity that it can produce the wealth and security needed for successful human rearing. But, society has its own interests and presses its own agenda on reproduction. Without stable family units in a traditional human society, there are a lot more orphans, a lot more starving women and children, and a lot more disorder. Disorder is bad for society, which depends on social harmony and cooperation to run efficiently. Society therefore, will promote the strategy that promotes the most social stability...
...wait for it...
...marriage, 1 man, and 1 woman. Who'd have guessed?
Let's face it, divorces and broken families are not good for societies. Women cheating behind their husbands backs, and trying to "Cuckoo" the nest isn't good for societies outlook on family units, so that female strategy is highly frowned on (though maybe not on the west coast of the US, which is pretty fucked up when you think about it).
Inversely, countless restless men, unable to find mates is also very bad. There's an ancient Chinese saying that says, "1 Bachelor is an annoyance, 1000 bachelors equals a war," and it's true. You don't want thousands of men, who can wield weapons, running around frustrated unable to find mates-- which is why male polygamy (strongest men take many wives) is also a recipe for trouble.
The 1 woman 1 man pattern solves a lot of problems. It's not perfect, and of course individual humans have tons of conflicting agenda-- but overall, it's the simplest, most effective pattern most suited for human reproduction. Humans are so complicated, so flexible, and often so stupid that we can and will sway from the pattern, and frequently.
But society will usually settle on it, and the majority of people will as well-- because for a lot of fundamental reasons, it's just the best pattern available. Also remember that society is the tool that makes the train of human survival go-- disobey society, and it'll bite you in the ass. You can try to use society to your advantage like with my earlier example, but there are definitely strong incentives to obey societies pressures-- like tax cuts and good relations with your neighbors.
BTW I LOVE my girlfriend, she's great ~~<3<3<3 If you want to hear my actual opinions/advice on love and romance, that will take another monster post...