Taken already lah! back off!

You should never tell someone that you like him/her when he/she already has a significant other. Whether expressly or impliedly. If you do, you:
1. put that person in an uncomfortable situation
2. put yourself out as a person who does not have much respect for the other person (i can think of a few names that would explain this sort of character but i shan't mention them)
3. put yourself out to be a person that do not value your worth very much.

Being the third party is never cool. being a destroyer of people's relationship is never cool, in the process you end up spoiling the friendship between you and that person.

So really, the next time you're out with someone who's already taken, NOTE THAT: he/she is TAKEN.