Given that Leviticus 18 is a list of banned sexual behaviours, you can argue that bestiality, incest and child sacrifice are OK because they are also in the list in Lev. 18.

That would only be true if my argument were that the things in Leviticus are not sinful. That is not my argument. My argument is that the fact Leviticus prohibits something does not mean that the prohibited action is sinful. It may still be sinful for other reasons, but it is not sinful merely based on the fact that it was outlawed in Leviticus. Child sacrifice, for example, is murder and an act of worship to another god, both of which are outlawed throughout the bible for obvious reasons. The presence of child sacrifice in the Levitical laws neither makes it more sinful nor less sinful than if it were absent from the Levitical text.