I went to America for 3 months and im really astonished by the culture differences. Opinions?

Im from Ireland and I went to America for 3 months to study as part of my course and I noticed that girls and guys dont really hang out together unless there dating someone in the group, or are interested in dating someone in the group. Girls and guys dont really hang out as just mates. In Ireland its not uncommon for a girl to go out to the pub or go hang out with a group of guys on her own just as friends and nothing more and its grand, but in America if a girl did that it seemed to be presumed that shes dating someone in the group or shes slutty. It was like what could a woman possibly have in common with a man? how could a man and a women just be good friends? I just found it all very strange, some of my closest friends are males we have a laugh together, we go out together and hang out. People are just people, you get on with some and you dont get on with others. Why was it such a big deal?