At least to some extent, we are a species with an evolved psychology. Like other animals, we have inherited behavioural tendencies from our ancestors, since these were adaptive for them in the sense that they tended to lead to reproductive success in past environments.
But what follows from this? It does not follow that we should now do whatever maximises our ability to reproduce and pass down our genes. For example, evolution may have honed us to desire and enjoy sex, through a process in which creatures that did so reproduced more often than their evolutionary competitors. But evolution has not equipped us with an abstract desire to pass down our genes. Knowing all this, what should we do? Well, we are not evolution’s slaves. All other things being equal, we should act in accordance with the desires that we actually have, in this case the desire for sex. We may also desire to have children, but perhaps only one or two: in that case, we should act in such a way as to have as much sex as possible while also producing children in this small number.
By all means, then, let’s use contraceptive technologies for family planning. This may be “unnatural”, in a sense, but so what?
Generally speaking, it is rational for us to act in ways that accord with our reflectively-endorsed desires or values, rather than in ways that maximise our reproductive chances or in whatever ways we tend to respond without thinking. If we value the benefits of social living, this may require that we support and conform to socially-developed norms of conduct that constrain individuals from acting in ruthless pursuit of self-interest. Admittedly, our evolved nature may affect this, in the sense that any workable system of moral norms must be practical for the needs of beings like us, who are, it seems, naturally inclined to be neither angelically selfless nor utterly uncaring about others. Thus, our evolved psychology may impose limits on what real-world moral systems can realistically demand of human beings, perhaps defeating some of the more extreme ambitions of both conservatives and liberals.