I agree for the most part with what everyone is saying. Unless they are proud cut, they don't have the hormones to act "stud-y". Some horses (mares or geldings) are just very territorial. We have one of each at our place... good Lord Katie kicks anyone and everyone that's next to her and Jordan bites anyone next to him. Lol It's just who those horses are.
Im not ever really a fan of turning horses out together (for many reasons and Im a former "show" girl so that could explain it) except for brood mares WITHOUT shoes (of course).
LOL... And yes, gotta love those good ol' broodmares they will teach some manners!!!!