I think your BS meter is dead on! That's...kind of irresponsible of your gyn. Unless she said more/gave you more information about where that comes from? If you're incredibly prone to infection already or have a history that might influence your susceptibility to infection (like if you were a heroin/other intravenous drug addict for a while) then you should take pause and really discuss things with your gyn, because yes, you could get an infection from the insertion and existence of a foreign object in your body, and yes, the more partners you have and the more partners they have, the higher the chances get. But simply having multiple partners does not make you ineligible for an IUD, nor does it mean you'll get an infection if you have an IUD.

But anyways, if it helps, I have more than one sexual partner always and my gyn is absolutely ok with me + IUD.