Korea is very dynamic and changing in many directions, but one thing is prevalent in some parts of the society is the segregation of the sexes. Essentially, guys should only have guy friends, and girls should only have girl friends. This is sort of a soft social norm, and not absolute, obviously it is possible to have friends of opposite sex, but one has to be careful and cautious. A guy seen to be talking to many girls or having many friends of the female nature will be automatically assumed to be a player or somehow frowned upon. A girl that is maybe tomboy-like with many male friends will not be in a good light either.
What will happen, especially in a college like Yonsei, is that there will be gossip, e.g. "That boy was talking to another girl yesterday, now he's talking to that girl. He's a player". Soon the guy may find himself unable to have female friends or date women, or get strange looks, wherever the gossipers spread the word, which in Korea can spread quite fast, even outside the university. This is what happened to my buddy who lost a girl he liked very much, as she rejected him due to undo gossip from Yonsei.
For Korean guys, maybe they will not get noticed, unless they are well connected or outgoing. For a white foreigner, everyone will notice, because in a homogeneous society where everyone is of the same race, he or she will stick out. And for foreigners, the glass is always half empty, meaning a foreigner will not get the benefit of the doubt and be scrutinized.
Personally, I had experienced some of this scrutiny. At college, I had the unfortunate experience of trying to make friends with some females from IYC (International Yonsei Community), a club that pretends to accept foreigners, but ends up alienating them due to cultural and language differences. Soon afterwards, I received strange repulsive looks and found people avoiding me, people that were one time very outgoing towards me.
I experienced some of the segregation in my classes, where clusters of girls will collaborate and share their notes, but when I try to ask if could get notes or collaborate as well, I'm rejected. Fortunately, even though there are minority of guys in the class, I started to organize them into our own male study group (and those that don't discriminate against males). In other classes, I found it easier to coordinate studies with either Korean males or foreigners, but not with Korean females.
After some rather painful moments, I became a bit more cautious and selective. The rare moment when meet a new friend, I try to meet off campus, so that I won't be seen and gossiped about on campus. If I do meet on campus, it is wise to meet in a mixed group.