One word, not more, LOVE.

If I understand the question, (and I'm not sure I do), you're asking which social structure is right for humans: polygamy (Multiple women for one man), monogamy (One woman for one man), or polyandry (Many men for one woman).

If you look at our cousins, the Apes, you see both polygamy, monogamy and a sort of all-around promiscuity that could be called polyandry, but really isn't.

Some interesting studies have been done comparing physical attributes (including size of testis) and social organisation. The result is that humans are either, by nature, polygamous or monogamous.

So we have a choice of either polygamy - one male breeding with many females, to spread his genes around, or monogamy - a couple, to which you could attach the words "faithful", "loving", and all those wonderful fuzzy words.

From observation, it would seem to me that females are built (physically and emotionally) to be devoted to one man. Men, however, seem to have an urge to mate with many other females. This results in a "harem" sort of living, with many females devoted to one man.

We know that in our current social stage, this doesn't work out too well, but it would appear that the (ancient) instincts still drive us toward that outcome. When or how this will/might change seems to be uncertain.
And, there are of course, always individual exceptions.