affair with very close fiend whom u cant marry

If your body, brain, mind, heart and conscious supports your sins & crimes then why one should bother the audience or the society at all? The unrwriten rules are for those who really care the bylaws of good behavior. If guilty feeling comes to your conscious then don't just ask for any excuse or help from the society, which ought not to forgive you in case you are caught. When some one avails an opportunity, definitly understands whether deserved or not. When you need a consent or acceptance from society that only shows your conscious is not coinsiding with your body and brain. Don't do these sins in all such situations said above.

No, I do not think it is a good practice to continue a serious relationship that has no end result for being barred by social rules. In such relationship both the partners will be on the looser side and neither the society, i.e., their friends, family members and neighbours will be happy. So, you need not be involved seriously in any such relationship where you can't marry.