Because, logically, it is against the Natural Law. It’s not how we were made to operate. Our reproductive parts were made to do just that, reproduce. Homosexuality, by its nature, doesn’t reproduce.
Most folks aren’t aware of what the Natural Law is. Let me offer a brief explanation. You probably have a car, right? Your car has an owner’s manual put together by the cars manufacturer. In that manual, it probably says you need to use unleaded gasoline in the tank to make it operate correctly. What if, in all sincerity, you came up with the idea that it would be better to put honey into your gas tank. After all, it’s “your car” right and you should be able to do with it “whatever you like,” right? What will be the result of you not following your manufacturers manual? You’ll be a pedestrian. Your car won’t work and you’ll have nothing but trouble with it.
The Natural Law is our Manufacturer’s (God’s) manual for human beings. Since He designed and manufactured us, He knows how we should best operate. If we deviate from that, we will have problems in our lives. Not maybe. But for sure, sooner or later.
If you would like to hear a good lecture on the Natural Law, I would invite you to listen to Dr. Charles E. Rice (professor emeritus at Notre Dame School of Law) at the following link:
alabamacatholicresources.com/Downloads/Natural_Law-1.mp3