I wouldn't call gay people a class of people.

They are distinct from heterosexuals and bisexuals, and statisticians note each as a class, group, etc. There is nothing demeaning in the statement.

Those of them who are non-affirming Christians acknowledge that they share in common a particular sin with each other

I don't assign harm based on sexual orientation - that something Christians (and other religious people) do in varying degrees.

find themselves convicted to believe that homosexuality is a sin... then that's between them and God

Except that it's not between them (Christians) and the biblical god - they bring it into the public spaces with acrimony and resentment. This is the reason suicide and homeless rates are so high among LGBT - it's religiously-based hatred protected by other Christians.

Because I'm not a social butthead I don't run around calling people reprobates and abominations on the regs.

Because I dropped religious morality (blind obedience to religious rules) in favor of human morality (reduce suffering), I don't suffering from that deficiency (needing to call anyone anything for how they were simply born).

because it would cause a lot of arguments if I were to tell every single human being I meet how sinful they are.

And that's the problem - judgment based on ideas that are over 2000 years old.