Alternatively, a person who goes all "free-fucking bonobos" has a better chance of producing a large variety of offspring, thus increasing the odds that they will be better equipped to survive and ensuring the continuation of the parents' lineage.
Now, it is true that children fare better if they have a parent that sticks around to raise them; but the other evolutionary behavior is to plant as many seeds in as many different kinds of mate as you can. The chances are that at least some of them will survive.
It's a mistake, of course, to believe that "raising well" leads to the institution of marriage, or that marriage and proper grooming are the direct result of some kind of superior evolutionary behavior. Marriage is a product of patriarchy, nothing more; and the value of monogamy follows from this. It has nothing to do with evolutionary behavior witnessed in other species.
In contrast to the false equation given above, it is actually the institution of marriage itself - a socioeconomic model instituted by those in positions of power in order to procure the most viable and fertile mates - that constructs our notions of what it means to "raise well." And this is merely for the purpose of perpetuating the patriarchy. It is entirely possible, in another set of environmental, cultural, or evolutionary conditions, that raising well means instilling in one's children the propensity for impregnating as many mates as possible.