Gee. Murder and robbery were illegal then, too. Should we legalize them, too? (Just a reasonable response to such nonsense to get them to admit that some of the things which were illegal should stay that way.)
I am telling you one of the arguments used and it is persuasive. In spite of my understanding that homosexuality is a sin and has serious consequences for a society that not only condones it but glorifies it, I still would not equate it with murder or robbery. These are the reasons we sometimes look like fools. Best not to make equivalencies like that - they don’t work.
In the meantime, many approved of slavery and while slavery was never condemned in the scriptures, it is very clear that the Commandments themselves would prohibit the “owning” of another human, and in particular, the treatment of such as substandard or, even worse, treatment that would not even be humane for an animal. We are commanded to love our neighbor but many seemed to have no issue with slavery.
It is a much more useful argument than yours. While I can hold strong to my views, I am not naive about the positions the other side takes and some of the points that they make and that some of them are, in fact, substantive, and attractive to many who consider themselves equanimous.