I am told by those who have children that one of the many sacrifices couples make to raise a baby is opportunity for sexual intimacy. I don't have kids, so don't know from personal experience, but it certainly makes sense. Between exhaustion, vehement interruptions and company in the house, it can be hard for a couple to find the time, privacy and energy to maintain their pre-parental levels of activity. I have had friends say that it seems very much like the baby is plotting to destroy its parents' sex lives. It has just occurred to me that in a sense, this could be very true.
Evolutionarily, there are many ways in which the interests of the parent and the interests of the offspring are aligned. The fitness of both are improved if the baby grows, thrives and go on to produce its own offspring. They share many genes, and anything that is good for the one is at least a little bit good for the other. But some things that are good for the fitness of the parents are a net selective loss for the baby. Such as having another baby come along too soon. The parents of course are equally closely related to all their offspring, and therefore will tend to distribute care and resources between their children in a way that maximizes the number of future grandchildren. But the baby is twice as closely related to herself as she is to her full sibling; she is much better off monopolizing her parents' time and resources for longer than they might desire. The parents' fitness is maximized by having an interbirth interval just long enough to get a good return on their investment in this offspring, without unduly diminishing their opportunity to have more children in the future. The child's fitness is maximized by having the parents wait somewhat longer, until the diminishment of their future reproductive chances for each additional day waited is twice the per day increase in her own fitness gain. The technical term for this disalignment of interest, appropriately, is parent-offspring conflict.
At first glance, the advantage in the conflict over the length of the interbirth interval would seem to be distinctly on the side of the parents, rather than the sessile, pre-sentient, altricail lump of chub, digestive organs and breathing apparatus. But oh, those breathing bits can very easily be used to make sounds. Sounds that communicate desperate dire need to protect and nourish the baby. Sounds that cannot easily be ignored. What harm if one screams just a little bit louder, a little more frequently, screams and cries with slightly less provocation, and makes the parents increase their interbirth interval just a little while longer?
Before you label me a conspiracy theorist, let me be clear. I am not implying that the babies of the world are 'trying,' in any intentional way, to deprive their parents of sex. They don't have to try. It comes naturally to them.