In other words, the contrast is between natural and unnatural, and also between heterosexual and homosexual sex. The documented medical consequences of engaging in such unnatural and unhealthy sexual practices bear this out. St. Paul makes an argument from nature. He is saying that the very notion of homosexuality is disordered and unnatural. There were plenty of negative repercussions for health with regard to sodomy before AIDS came around.
Sin is always dangerous.
God tries to spare us of its consequences, but we are too dense and rebellious to accept Him at His word, and we insist on playing with fire, defying even nature itself, if need be. In other words, there is an ontological, created reality and natural order beyond mere physical pleasure, which must not be violated. Certain things are wrong by their very nature. So glorify God in your body. So heterosexual fornication and adultery are just as sinful, wicked, reprehensible, and unnatural as homosexual sex.