Because the good book tells us so. Where it is defined (ex: Georgia) this 'crime against nature' has been legally defined as 'any sexual act involving the sex organs of one person and the mouth or anus of another'. In other places it has been deliciously left undefined so that the court can include any sexual act deemed to be "unnatural" or "immoral" under this umbrella.
Welcome to your post-Roe world.
Please note that per the GOP sex with children is not an 'unnatural' or 'immoral' act (ex: Tennessee House Bill 233 & pic related)
![](https://i.imgur.com/OoW7KmL.jpg)