6 Sour... Lime

How close can a relationship of a boy and a girl be? I always think that if a boy and a girl go out for a drink or to do something with just the two of them, there is something going on between them... like either one has feelings for the other or both do... It's not that it is wrong for a boy and a girl to hang out alone, it's just that if one of them has a boyfriend or girlfriend, then it would be kinda wrong to do so. But I still don't get why would a guy, who has a girlfriend already, would still go out with a girl alone... what do they have to talk about that have to be just the two of them? Why cant they invite more people to join them or something... If they are just going out to hang and talk about random things, then why do they have to do it alone, if they are doing it alone, wouldn't that means they want to be alone with each other and learn more about the other?? Like unless the boy and the girl are extremely close, let say, extended family (non-biological family), then it would OK to talk about their issues and stuff. But it's not about issues or problems... then I don't see the suitable point for a boy and girl to hang out. Of course, hanging out, meaning calling someone out and stuff... not like the two randomly met each other at some place... It's the calling each other out that's considered to be wrong, in my case...