Presumably art would be considered a more complex, symbolic, and layered act. But good sex is a symphony of experiences infinitely complicated with meaning, rich and unpredictable, as capable of disturbing and illuminating the individual as any formal work of art, as memorable, as fleeting. The real point being made is that sex is low because it's universal. After all, any chimp can fling paint. In the grand scheme of things sex is nothing beside the more publicly applauded accomplishments.
Sex is the ultimate ephemera, a phantom.
But our culturewide aversion makes sex more, not less, real. Refusing to look at an illusion gives an illusion body and strength, gives it power. If you want to make a mirage go away, walk toward it. If you turn and run, the lake gets bigger and the palm trees more inviting every time you look back. Sex is as important, in much the same way and to about the same degree, as what we eat and how we sleep.