The two of you can be polyfidelitous without everyone in the relationship needing to be. The relationship isn't if not everyone is, though.
Similarly, if the BF is mono, for instance (it's just an example), that doesn't make the whole relationship mono. But he doesn't stop being mono because his partner isn't.
Anyway, poly works as a general term. You don't necessarily need to specify you are not interested in new partners, unless it's relevant at the time you mention it