You’re using an argument that I’ve developed recently in a more general manner.
Usually the people who try to say that homosexuality is morally OK use relativism as their justification - ie, you can’t impose YOUR value system on others (which is what they are doing with their relativistic value system - but that’s another argument).
They reject the idea of moral absolutes, especially those written down in the Christian Bible.
However, they don’t really think that way. They believe THEIR value system is superior to yours, but the only way that ANY value system could be better or worse than another is by comparing both to an external absolute standard.
So, they either have to admit that their value system cannot be “better”, or they have to accept the existence of an absolute. If the latter, that absolute has to be referenced and agreed upon by everyone. And where are we to find this? aha!
The relativism argument will be used to flush out all traditional values so that the vacuum can be filled with leftist values. It’s a technique used by the left, especially by leftist profs on students.
As well, the left is quite use to living with contradiction. Martin Heidegger said that life is essentially, at it’s deepest level, contradiction. This was taken for profundity rather than something to be avoided.
When one of them rails against somebody over being racist or sexist, just ask them "why do you insist on imposing your value system on X?", pointing out that the position that racism is bad is itself a moral judgment. Watch their heads explode.
That's it in a nutshell. That sounds like a very good technique. Thank you