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

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

That is entirely something আপনি should be discussing with your Significant Other, and depends on the terms of the relationship. দ্বারা all means, আপনি do you, and don't let anyone dictate who আপনি are বন্ধু 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 আপনি were up front about it and now there should be no confusion.