Stigma against fornication is a very basic natural human response.
For example, let's assume (for some weird reason) that you are okay with your wife/gf sucking, rimming and eating cum from many other men. Now while you are kissing her, how are you going to feel?
Now expand this idea, assume you are in a poly relationship and you have a child, your wife sleeps with many bulls, rims their butt-holes and then kisses your child. How are you going to feel for your kid?
Expand more, now assume you are brought up by parents who practice poly-relationship, which means your mother licks butt-holes of many other men on a regular basis. How does that sound?
If you try to fiddle with this basic feeling of repulsion by normalizing fornication and whoring, you are definitely doing something wrong.