Newt was born inter-oriented-- he has both a knot and a self-lubricating ass. Rather than forcing surgery on him, his parents let it be and allowed him to decide what orientation he was. Newt struggled with it, never able to find a partner who was willing to accept both his alpha and omega nature.

Then he finds alpha!Credence and omega!Graves. Not only are both 100% okay with Newt's mixed orientation, but they welcome him into their bed and their relationship. Happy, fulfilled threesomes all around. Specifically with Newt fucking Graves while Credence fucks Newt.