God has a perfect plan for the universe and has made us and all of the creation to fulfil His design. We must act in accordance with this plan. We can see that to fulfil our God given purpose of reproducing and populating the Earth we should form heterosexual relationships and raise children. Thus same-sex relationships are simultaneously sinful and irrational. 

Aquinas wants us to use our reason to see what kind of lives are good for humans. He thinks that good lives contain friendship, a place in a good society and are illuminated by the right use of reason. Marriage is another core human good, and in fact Aquinas referred to it as the ‘greatest friendship.’ Marriage, for Aquinas, combines monogamy, love, and mutual regard, and it is in turn oriented towards the goods of procreation and the raising of children. Sex- within the confines of heterosexual marriage and which is supportive of the goods of marriage- is permissible and can even be described as a moral good. 

Aquinas adds, however, another requirement for sexual acts to be moral: they must be potentially procreative. Hence, only heterosexual relations involving vaginal intercourse are acceptable; non-‘generative’ sex acts are immoral and irrational.

He said that:

It is not the function of the State and the law to constitute themselves guardians of private morality

-Albert Ellis -psychiatrist