And I don't mean boyfriends... I mean BOYS who are Friends... I was really BAD at this in my formative years. I think there is a good reason for this. I was an only child (no brothers) and my dad was, for the most part, absent, even before he died. Oh, sure, I had boy cousins, uncles... neighborhood kids... but I really had no awareness that boys were people (who knew?). I usually stammered awkwardly, or flirted badly (as in with poor skill, not as in overdoing it, though if I added alcohol, it tended to cross from the one to the other and there was a time I thought this meant I was succeeding.)
There were a few exceptions though, and I have analyzed it (blame my Virgo Moon). The boys I was capable of being great buddies with, without awkwardly walking the 'der, what do I say, am I supposed to flirt now?' thing, had SISTERS and tended to be raised by ONLY their mom... they were USED to being doused in estrogen. I have several male friends with whom things were FAR easier to find all the lines because THEY KNEW HOW TO TALK TO GIRLS! (so I didn't NEED to know how to talk to BOYS).
Getting along with different types of people TAKES PRACTICE. Oh, sure, some people just have more social skills (or enough drive to develop them, or parents better able to teach them) but people exposed to a great variety of people will be better able to deal with them LATER.
I think in college, when I had my first 'exclusive' boyfriend, I managed friendships with HIS friends, and LEARNED how to do it... and in my 20s, nearly all of which, I was with my husband, though we didn't marry until I was 25, the OTHER men I got to know were ALSO just friends... I had finally mastered it... But it took security (on my part, this took the form of 'already taken, not looking') and opportunity (work or school environments where I MET and interacted when I wasn't looking... see, that LOOKING interfered a lot when I had no clue how to talk to boys and desperately wanted some media version of 'happy ever after' type love (STILL can't read romance... misleading tripe)...
Now for the APPLICATION...
Want a rationale for why your character gets along better or worse with some group? Need a rationale for a tic or a quirk... a set of misconceptions? Give them a history! A lack of experience or a bad experience can lead to a whole mess of faulty attributions. This is especially true where there is an unfamiliar GROUP and a character's OWN group talks about them in certain prejudicial ways. But for something as simple as “can she talk to boys?” this is a great way to set up a realistic route of ineptitude.
If a character acts a certain way... has certain social skills or LACKS them, this is one of the many options for explaining and expanding on that... for REALLY getting your characters.
As for REAL Life...
Get to know people from as many walks of life as possible. The faulty attributions are only useful if you KNOW what is faulty attribution and what is real. Cultural differences really exist. So do stereotypes. Learning the difference is how you can keep from walking the wrong side of that line.
So there you have it... Advice a la Tart....