Why is gay marriage or sexual relations bad?
First, it’s important to note that there can be no such thing as “marriage” between two members of the same sex. This is because marriage by nature is a union that is ordered toward the procreation and education of offspring, which necessarily involves both male and female. So, to talk about “gay marriage” is an unintelligible phrase, akin to a “square circle.”
With regard to why sexual activity among members of the same sex is bad, we must look to the nature of sex and what is good and bad given our nature as human beings.
What is good for us as human beings is determined by the ends of our inherent capacities. Just as it is good for a pencil to write, and it is good for an oak tree to sink roots deep into the ground and take in nutrients and water for its growth, it is good for a human being to know truth and avoid error, to live in society, to propagate its own species, to take in nutrients for human growth, etc.
The good in the sexual arena, therefore, will be the achievement of the ends toward which our sexual faculties are naturally ordered. When we look at nature, we see that our sexual faculties are ordered toward two ends: procreation and unitive love. Therefore, the good in the sexual arena is the achievement of procreation and unitive love, which are its natural ends.
Now, since we’re rational beings and therefore ought to do good and avoid evil, it follows that we ought to use our sexual faculties in ways that are consistent with their natural ends. To do otherwise would be to behave in a bad way, given our nature as human beings and the nature of our sexual powers.
Sexual activity among members of the same sex is therefore bad for humans because it uses the sexual faculties in ways that are inconsistent with their natural ends. Under no circumstances can such behavior be approved of or promoted. For this reason, the legalization of unions that involve sexual activity among members of the same sex (“gay marriage”) is bad.