You may be the husband, but the children may not be yours.

Because reproduction does not limit to selecting only one partner. You want a healthy or fitted partner that will provide you an increased opportunity to carry your genetic lineage.
But this is what she wants, too.

A new study made at Florida Atlantic University suggests that the human male has developed mechanisms to ensure the transmission of his genes during post-copulation as well, a biological function called "sperm competition", which would be "the inevitable consequence of males competing for fertilizations."

Sperm competition is wide spread amongst promiscuous species (like many insects and mammals) but it seems quite weird in a monogamous species like the human one. But in reality, biologists believe that strictly sexually monogamous species are almost non existent. Even in species that are regarded as examples of fidelity, like many birds, females have "affairs" (extra pair copulations) and humans are no exception as we are a species recently evolved from highly promiscuous chimpanzee-like species.

But in a species like ours, where the male invests all his resources in raising children inside a monogamous couple, spending them into genetically unrelated offspring means a biological disaster. "Competition may also affect sperm count," say the authors.

For example, when men spend more time away from their partners (when their partners could get the opportunity to mate with other males), the number of sperm cells per similar sperm volumes rises sharply.

In one research, phalluses after molds of human penises removed a sperm-like substance from an artificial vagina, pointing that the penis developed its shape to act as an anatomical squeegee.

There are also sexual behaviors pointing to sperm competition. Women report that men thrust more deeply and quickly into the vagina after allegations of infidelity, a mechanism researchers believe is directed for sperm removal.

The authors believe that not only the increase in sperm cells after period of separation is a sign of sperm competition, but also their greatly increased libido in the same situation: the male wants to copulate as soon as possible and as much as possible as insurance against possible extra-pair fecundation. "Sexual conflict between males and females produces a coevolutionary arms race between the sexes, in which an advantage gained by one sex selects for counteradaptations in the other sex." said the authors, Todd Shackelford and Aaron Getz.

It would be interesting to see in future studies if females developed mechanisms for increasing retention of sperm, after being inseminated by males with the best genes.