All these people who support “gay” marriage always talk about happiness. That’s the best reason they can conjure for allowing such nonsense. Truly, there is no logical reason to support such deviancy.
Thomas Aquinas did also but, of course, not in this regard.
Happiness is the one thing that people can agree on. They all want as much of it as they can get. The issue is what happiness consists of. A lot can be said about a person based on what they choose as their ultimate happiness.
Thomas' conclusion was that human happiness consists in God alone and that those who pursue other things for their ultimate happiness find their ultimate disappointment.
Since you are a self proclaimed "conservative in training", I invite you to read the Summa Theologia, the first part of the second part, question 2 in its entirety. It is available free on line. Don't be put off by the style of the "disputed question", it's how science was done in the middle ages.