Hello. So to have kids we have to have sex. So the real question is what is the purpose of sex. Sex is only and realistically for the reproduction of the species. Some people believe that the purpose of sex is pleasure. This is incorrect. You can many different ways of getting pleasure in your life. You can only truly have kids through sex. Sex is and always has been and always will be for the reproduction of a species. The End.
That is not just as easy as you try to depict it here. Purpose is not there in itself, it needs a subject. Nobody ever decided sex is needed for the reproduction of the species. In the evolution simply the beings that had sex lived on and gave these genes on and those who haven't aren't around anymore. If you say the purpose of sex is to let the species live, it sounds like someone planned or intended it. But as water does not intend to reach the ocean but just follow gravitation, sex developed based on natural laws, not intent or purpose.
Yeast has not the purpose to create alcohol and create pleasure for people who consume it. Still that's what happens. Sex didn't developed because of reproduction of the species, but said reproduction led to the proliferation of sex. Now yeast is used to produce alcohol for pleasure and sex is used for pleasure too. It doesn't matter for that, what the reasons for their development were.
Creation and destruction is basically a universal truth when it comes to living things and even beyond that, since we wouldn't exactly call planets themselves living. A species that doesn't reproduce goes extinct eventually. I don't know of any living creature that doesn't reproduce in some form. Humans have their own version, but our's is designed the way it is, to make the process as pleasurable as possible, so we take part in it more often, which increases the chances of reproduction. Nature is pretty efficient, and wouldn't go to the trouble of making you more sensitive just for the heck of it, especially considering the world itself is not a sensitive place. When your capable of being able to participate in things like video games, theme parks, etc, she has to try and make reproduction more enticing or the longevity of the species is unlikely.
Sex is necessary for reproduction and yeast is necessary to create alcohol. You can use yeast for other things, just like you can use your reproductive organs for other things, but who would prefer that over the pleasure of alcohol and sex if it was an option? Some people have kids, some drink, and some do both.