If someone is obviously taken, just leave that person alone.

If someone is obviously taken, just leave that person alone. Don't tempt them to do something they'll regret. Don't hit on them, or send them cute little signals that you're interested. That person is taken because they like or even love the person they're with. Who are you to come between that commitment? It's not just right. Go find someone else to hit on. There's plenty of people out there for you.