For me polyandry means you have several primary partners that are all male, polygyny means you have several primary partners who are all female, and polygamy includes both, plus cases in which your primary partners are mix-matched (some male, some female, some genderqueer, you name it).
I don't think it changes a thing whether they have other partners or not. I'd consider myself polyandrous, even though both my partners are poly.
I think males are often okay with females having other partners as long as they're female for a mix of reasons:
- insecurity: they could wonder "why am I not enough" with male partners, but with female partners, it's easier to grasp: it's more obvious to them how a female could bring you things they can't. And on the other hand, they also know they can bring you things females can't, so it's safer for them as well. (Note that I believe this is true for ANY partner - they bring you different things from the next - but I think it's just easier to wrap your mind around it if there is a major difference such as gender. Especially for a mono person).
- they think it's hot
- in the case of straight guys, it might also be that they want you to have partners they are also involved with, or are able to get involved with potentially.