The Best Friend’s Beau: Should you be his friend too?

We’ve all been there; your best friend is intruducing you to her new boyfriend for the first time. You play it cool because you want him to like you but at the same time you want to be the mysterious cool girl. But he is a little too friendly. The one that doesn’t stop talking to you and is trying to get on your good side because that will earn him bonus points with the girlfriend. You go with it but then it gets to the point of is he hitting on me or is he just that friendly? Or even worse, why does he need my approval that badly? Then there’s his polar opposite; the guy that is a total dick towards you because A) he literally doesn’t care or B) he doesn’t want to come off as hitting on you. Unfortunately I’ve experienced both. Between needy and completely iced off, there has to be an in-between. There has to be a middle ground where the best friend and the boyfriend can co-exist. And if there is such a place, what are the rules?
Well I am no expert but I have picked up some tips along the way. I’ve made it a rule to not even meet the boyfriend unless I know it’s serious. Girls go through so many guys that there is no point trying to know the flings if they’ll be out of your life by the end of the week. It’s not worth your time or effort. But if a lucky one does make it to the introduction, how well should you get to know him then? Is it possible to be both of their friends without getting in the middle or betraying your best friend. One of the main issues I’ve dealt with is it okay for him to talk to you about her? And based on experience, I’ve decided that sure, as long as its platonic and only light subjects. If he starts complaining about her or trying to know you deeper, then that is when you bow out. You don’t need to complicate things.
At the moment I have a best friend who’s boy is trying to know me for her sake, because he wants to be a part of her life, while I have another best friend who’s boy only talks to me when he has to, like when all of us are together. And I honestly prefer the latter. Unless we were friends before, I don’t need to be friends with you. Yes we can be friendly and talk when she’s around but other than that I’m good; I have my own boyfriend and I already know a lot more about your relationship than you think I do, so no, I don’t need to be your friend too. That sounds harsh but it makes sure things don’t become complicated and you don’t become like a child in a divorce when they break up. So I guess the answer to my own question is yes you can be friends with your best friend’s boyfriend but only in the way you know a classmate or a co-worker, nothing more.