To begin, it is important to understand that evolution, or accidental existence, by its definition and nature, cannot include concepts such as planning, thought, or even the ability to determine a need for something. All of those things require intelligence.
Therefore, without accepting the existence of a God, it is impossible to explain why - and how - the act of sex creates a physical sensation of ecstacy.
Certainly, any "designer" would understand that in order to survive, creatures must procreate. And to insure they do so, it would be necessary to instill such a sense of pleasure derived from the act of sex. But my point is this - it takes rational thought, planning, and determining a need for the sense of pleasure. Without intelligence, "nature" and accidental existence would not beget such pleasure. Not being able to think, plan or determine, nature could not possibly, and by sheer accident, instill in us the sense of sexual pleasure.
Some might say that the ecstacy evolved because of the need for it. But that raises two questions:
1) how does nature, unable to think or plan, determine such a need, and "evolve" it, and
2) if it evolved over time, how did life survive long enough to develop that sense of ecstacy? Without it from the start, it is unlikely that creatures would procreate.
Certainly, evolution can explain many things. But what each of us should think about are those things that evolution cannot explain.
Take, for example, the intricacies of coexistence. Needs between two or more living things cannot be explained through evolution, as one species has no way of "knowing" what another needs. Here is an example:
Birds eat raspberries. But they cannot digest the seeds. Instead, those are excreted while the birds sit on tree branches. This allows the raspberry seeds to produce new plants because a) the seeds were not digested, b) the seeds were deposited beneath branches where they receive the required shade for the young roots, and c) the seeds are deposited with a glob of bird manure, to fertilize the seeds. An intricate series of instances that produces an end result. And because of it, the birds are helping to produce their own food supply.
This raises questions, for those who do not believe in a Divine Intelligence. One such question: why is it that the seeds are not digested? Surely evolution, without intelligence, would insure that the digestive system of birds would digest everything they eat. Considering all of the separate things that must occur, in concert, in order to continue the cycle, and each must occur without the knowledge of any of the "players", it is difficult for any thinking person to reject out-of-hand the concept of a Divine Intelligence.
Of course, as a believer I always like to ask non-believers the following two questions:
1) Since only living things can evolve, how do you explain the existence of all the non-living things, like rocks? Since they could not evolve, the only other explanation is that they were somehow created. By what? Why? How?
2) Assuming evolution explains how humans got here, how do you explain the very first living thing? Being the first, it could not have evolved from anything, because it was the first. So, where did that first living thing come from? How did it become alive?
In any case, I always come back to the wonder that is the ecstacy we feel when we complete the sex act. Such a thing is something that has a defined purpose, but not a true need except in the context of propagation of the species. And nature, or accidental existence, being unable to think and therefore unable to care, would not provide such a solution. The fact that 96% of all species that ever lived have already become extinct proves that nature does not care if we survive or not. Providing a solution for a purpose is something that requires intelligence, thought and planning.