Is it okay that I give my best friend oral even though they are in a relationship?
Are you sure you're doing the right thing here?
Yes, for sure!
You shouldn't be having sexual relations with someone who's already in a relationship! It's wrong to make someone cheat on their partner. One shouldn't ever cheat.
My best friend and their partner have this cool arrangement.
At least you are all consenting and aware of the relationship. Still, this will usually end up in drama and someone is going to get hurt. Keep your romances exclusive and find someone else to engage in sexual practices with.