Is it ok to cuddle with a friend of the opposite sex if 당신 are in a relationship with someone else?

I think so, but only if there is nothing sexual going on, like groping 또는 kissing.

That is entirely something 당신 should be discussing with your Significant Other, and depends on the terms of the relationship. 의해 all means, 당신 do you, and don't let anyone dictate who 당신 are 프렌즈 with, but do keep in mind the feelings of your S.O. and be courteous and mature about your habits. If s/he is uncomfortable, allow them to express that and don't get defensive about it. Just listen. If s/he has no problem with it, then at least 당신 were up front about it and now there should be no confusion.