At some point in most mens' lives, they will have been dating a girl, and had one of the following thoughts:

"Aren't her friends annoying?" "Don't her friends seem slutty?" "Why is my girlfriend friends with these people?"

Now lets say we unplug ourselves from the matrix for a second and imagine you're dating one of her friends. Now the crazy thing is, the girl you are dating would become one of those annoying slutty girls. Not so special now eh?

There are a few points here. The first and most important is that many men in LTRs have the tendency to slap the rose-tinted glasses on, crank up the oxytocin, and begin their betasization by 1000 concessions program. Why? Because she's SPECIAL and BEAUIFUL, and most importantly, SHE'S different from the rest! Sorry to break it to you buddy, but once the chemical reactions die down, you will find she's just like her friends, and if you wouldn't date them, you shouldn't date her, because they're her most powerful and consistent influence.

Now leading on from the influence point. Your lovely girlfriend will go out and spend time with her slutty friends, and therefore will engage in the same activities as them. Now women may not be able to design a machine or fly a plane or parallel park like a man can, but one thing they absolutely excel at is thriving in group settings. The female brain is hardwired to be able to survive and thrive in group settings, because many many years ago, when we lived in huts and threw spears, she depended on this for survival. She needed acceptance of the group to access its resources. Why's this important? Because your girlfriend will seek the acceptance and approval of her slutty friends above all else, above her 'love' for you, because that's just how her brain is wired. So, she will grind on that guy in the club, suck that guy off after the club, do double triple vodka shots, smoke that weed if pressured, because her friends validation is more important than yours.

Another side note for you, girls' night is never girls' night. Women group together to find other guys safely. As soon as a group of girl meets a few guys the immediately. discuss and start allocating the guys, a sort of "who gets who" thing. If she clubs with her friends, soft next or suffer the consequences.

So, to conclude, knowing her friends is ESSENTIAL to a healthy long term secure LTR (if that exists). You have to be able to trust her friends and know they'll keep her behaving responsibly, and stop her from slutting around, as she's probably more similar to her friends than your loved up mind could know.

A final side note, if most or all of her friends are in LTRs, she's more likely to stay with you, as the social condition for acceptance has moved from slutting around on girls' nights to having a boyfriend or whatever, and girls will always follow what's encouraged and accepted by their friends.

TL;DR: if her friends are sluts, she probably is too.