If humans were removed from the world, the ecology would suffer no catastrophe, in fact it would probably be better off. Like frogs, humans are in the world, unlike frogs, humans are not of the world. The creation waits in eager expectation for the sons of God to be revealed. For the creation was subjected to frustration, not by its own choice, but by the will of the one who subjected it, in hope that the creation itself will be liberated from its bondage to decay and brought into the glorious freedom of the children of God. We are the gardeners, it is the garden in which we learn and demonstrate stewardship.
Frogs have a function, but humans have a destiny.
Unlike frogs, humans have no need to replace themselves, not only because we perform no natural function, but because we are immortal spiritual beings. Humans have sexuality because we are animals, and animals have sexuality. Therefore, many humans mate and have offspring. While it is true that much of our sexuality results in rape, adultery, betrayal, abortions, abused and abandoned children, battered spouses, and prostitution, these vices are only possible because we are capable of virtues. And herein can be found the true purpose of human sexuality.