Someone tells you that you and a certain good opposite-sex friend of yours are "cute together" when they watch you interact? Is it normal to just ignore the statement? Or by saying nothing, both have already acknowledged that this in fact was a correct observation, but is taboo to bring up further, since one party is in a relationship?
What would you do if you were in the single girl's place? Or guy who is seeing someone else?
That depends. If it were an acquaintance, I'd probably blow it off. If it were a close friend of the girl, I'd probably look into it as "she likes you and wants you to ask her out" or something along those lines.
This is kindof a girl thing. I'll usually do this only if I know one of the people is interested in the other. It's really hard to just come out and say "I'm interested in you", but if I as a friend say "Wow you guys would make a great couple!" then it becomes almost a fun thing - the person that's interested can be like "haha can you believe him?!" but still get to see the other person's reaction. It's like having a wingman.
If the other person is in a relationship that's different. You want to respect that, but at the same time, at your age, nothing is set in stone unless there's a marriage involved. It's OK to show interest and let someone know "hey I'm interested in you but I respect that you're in a relationship". It's way more dishonest to be like "yeah lets just be friends" when really you're romantically interested in that person. Just be honest and clear so no one gets hurt, espescially yourself.