There is an answer, and I'll give it to you: because our culture places a curious and poignantly hypocritical stigma on sex-positivity.
To answer the spirit of your question, because there have been hundreds of millions of years of evolutionary pressure positively selecting for sexually-reproductive organisms to be terribly interested in acts leading to sexual reproduction. Any indictment of that is not only misguided in the extreme, but an indictment of most terrestrial life as we know it. Put that in your "no horny" pipe and smoke it. (Not YOUR pipe, but you know.)