OK, natural selection is the retroactive mechanism in evolution. But sexual selection is proactive, it's all the singing birds, and the stubborn determination of people to control their environment and to change the environment of other living beings. If it wasn't for sexual selection, an extra layer of life would be taken away from evolution. In fact, sexual selection changes the pool of properties of living things, be them genes or behavioural patterns, more than natural selection ever could. Natural selection is a rather rudimentary process by which the utmost unsuitable, on the fringe organisms and their genes get removed, those that have ceased to be complementary to their environment. That leaves all those others, and there are quite a lot in between. Essentially, contrary to what may appear like an obvious "designer", natural selection does not shape perfectly suited organisms to their environments. It removes the unsuited ones, but of those which remain, they present a gradient of survivability factor.
That is, they are all equally suited to survive and reproduce in their environments, but not all do. Unlike natural selection, sexual selection actively seeks the very best characteristics. The best characteristics to suit an environment, and in humans' case, the best characteristics to change the environment. The more we alter our environments, the more we diminish the role of natural selection, in favour of sexual selection. Essentially, otherwise naturally occurring selection pressures (such as disease or lack of resources) have been taken over by artificially occurring ones. Therefore, it is safe to assume that there is a negative correlation between the intensity of natural selection versus sexual selection. In a case where sexual selection hardly takes place, natural selection will appear more predominant, since it will take place relatively more often, and hence a species will be merely shaped by characteristics deemed suitable to cope with certain natural selection pressures. However, in a case where sexual selection is so active that it leads a species to progressively alter its natural environment, consequently natural selection will be less prominent in the species' evolution.