You would think that this would be quite a simple question to answer; I use to think it was but now I am not so sure.
If you know that the person you want to ask out is in a relationship and they show no interest in you I think you have no place in asking them out. In that respect I have always felt that someone in a relationship is “off limits”. At the same time if you feel that they’re unhappy in their relationship I still believe you have no right in asking them out.
However the line blurs when the person in the relationship shows keen interest in you. (Which happens to be the case for me at the moment, at least I think it is).
If someone in a relationship is showing interest in you, what is appropriate behaviour on your behalf? The answer can be somewhat clouded if you of course already have feelings for them yourself.