My arguments, though parallel with the Church's, are really societal based: the belief in male-female lifetime unions for raising their offspring, and encouraging our youth to follow that path for fulfillment and purpose in life.

Of course, there are others who believe in, e.g., non-structured societies where no one forms couples, people engage in sex acts with whomever, and offspring are raised by the tribe (like many animals).  I have no experience with such a society, but I can see where someone might find the idea of such a society appealing.  Thousands of years of human history have proven that male-female unions raising their offspring create stable societies with the best outcomes for children.  

Societies that have incorporated same-sex acts as normal have ultimately dispensed with acceptance of them.  Again, I think the AIDS crisis shed some light on what may have happened that changed those societies' views.