Now I know most of the frequent readers and contributors to this site are not fans of philosophy, that is, the play of words as it has been adeptly described, but that my friends will not stop us from time to time sharing philosophical musings.
Here Schopenhauer is talking about love or rather here specifically sexual relationship, if I can call it that. He writes
First of all we have to remark here that by nature man is inclined to inconstancy in love, woman to constancy. The love of the man sinks perceptibly from the moment it has obtained satisfaction; almost every other woman charms him more than the one he already possesses; he longs for variety. The love of the woman, on the other hand, increases just from that moment. This is a consequence of the aim of nature which is directed to the maintenance, and therefore to the greatest possible increase, of the species. The man can easily beget over a hundred children a year, the woman, on the contrary, with however many men, can only bring one child a year into the world. Therefore the man always looks about after other women, the woman, again, sticks firmly to the one man; for nature moves her, instinctively and without reflection, to retain the nourisher and protector of the future offspring.
Accordingly faithfulness in marriage is with the man artificial, with the woman it is natural.
In his book, the monogamy myth, David Barash looks at extra pair copulation among animals and reports it is prevalent in almost all species. He reports that among birds, when and if a bird pairs with a different male, it does so secretly so as not to jeopardise the relationship with the regular male.