I dated girls like this before. Turns out, they weren't friends - just options. They'd claim otherwise, say they're just friends, they've known each other for a long time, they have lots of guy friends, they're sociable people, they don't get along with other girls, if you don't accept that - you don't accept them... Basically, fighting tooth and nails and coming up with excuse after excuse to keep seeing these options in one-on-one situations. Fell for it big time... Only to find out all these girls were players. That's why they were so passionate and defensive about it.
Always trust your gut. If something seems amiss, its usually a warning sign.