We're all born with an expiration date. No one lasts forever. Life is nothing but a grace period - for turning the best of our genetic material into the next generation. The data of life is transferred from parent to child. That's how it works. But we have no heirs, no legacy. Cloned from our father, with the ability to reproduce conveniently engineered out. What is our legacy if we cannot pass the torch? Proof of our existence - a mark of some sort. When the torch is passed on from parent to child... it extends beyond DNA; information is imparted as well. All I want is to be remembered. By other people, by history.
I don't doubt for a moment that the tradition of not seeing the bride for a day before the wedding exists for that reason. It's her last chance to bear someone else's child. It won't be until night that her husband will sow his seed. Until then, it's the first guy's sperm and no competition.
In this day, you simply must have paternity tests conducted for every child you have. Evolution has produced many reproductive strategies, including infidelity. Females are by far its stealthier and more damaging practitioners.