The reason some sins are not explicitly mentioned in the Bible is that they were considered so taboo that they were not to be named; rather an indirect term was used to convey the meaning. We say something like “sin with oneself”, for example when we don’t want to come right out and name it. But the ancient Hebrew people were totally disgusted by such practices–so much so that to even name them was anathema to them. So, there are no explicit references to this sin, but it is implied in the word “impurity”, as I cited in the scripture verses in my first post.
For those who won’t stop this practice unless they see an explicit Bible verse condemning it, there may be no argument, no matter how reasonable, that will convince them they are committing a grave sin. Sacred Tradition, sad to say, means nothing to them. Besides, they want to engage in this sin, so they are going to find excuses to do it. Still, we can try to tell them the truth. But it’s up to them to listen and act on the information. Apart from praying for them, there’s nothing else one can do.