Seeing as how a no label relationship entails zero commitment, any of both parties can essentially flirt and hit on whoever they want; there are basically no rules for actions. You have every right not to be monogamous, as do they, and none of you can say anything about it.

What does a no label relationship mean?

When people talk about labels in a relationship, theyre usually referring to terms like dating, in a relationship, boyfriend and girlfriend, and the like. A relationship without labels is any arrangement between two people who are choosing not to adopt any such terms to describe their relationship.