Where in the Bible does is say that Leviticus 18 is exempt from being written off?
Seriously. Where does your claim that Leviticus 18 holds more water than the rest of the book come from?
Haldo: If you're genuinely curious, then for centuries now, exegetes have taken examples such as Mark 2 and Acts 10 (to name just a couple) as countermanding many Mosaic commands and proscriptions—dietary and otherwise. However, the Marcionite idea of supersessionism—that the OT is simply a dead letter—got thoroughly refuted more than 1800 years ago, along with Marcion and his anti-Semitic crowd.
So which OT injunctions are still in force, and to what degree? That's something that theologians have done a lot of heavy lifting to answer, from the earliest writings up to the present. Renewed NT admonitions against previously banned OT practices carry special weight, of course, but there's also been a ton of textual analysis, historical inference, and applied moral theology (see Aquinas, Tertullian, Philo, etc.).
You're perfectly free to contest the Church's reasoning on these issues, but it won't do to act as if no one's done