Off limits: When is "taken" not really "taken"?

I've had a few people ask me how do they deal with situations where the person they like is in a "complicated" relationship.  Sometimes the other person has a significant other who is hundreds of miles away.  Other times they have an on again off again relationship with someone who they just can't let go.

When relationship statuses change with the wind, how do you really know who you should deal with and who you shouldn't?

One of my homeboys has a "graduated" system:

I classify it on different levels and this is from minor to major...{just having sex} (self explanatory), dating(fair game), engaged(give or take, depending on morals). And Marriage (off limits)

This is why things are so confusing.  You don't know what situation a person is in.  If they are giving you a lot of attention, you often justify it by saying "well if he/she were happy at home, then I wouldn't get all of this attention..."  Eh...not necessarily the case.

I say never give your time and attention to someone who is involved with someone else.  Meaning someone else is laying claim to him/her or vice verse. 

But if someone hasn't made a commitment (marriage/engagement) and decides to leave a situation to pursue you, then by all means, let the situation play out. And I do mean LEAVE the situation.  And allow them to leave without any coaxing or interplay from you. Let it be of their own volition because that relationship was not for them.

I agree with my homeboy to an extent... I think Engagement and Marriage are both COMPLETELY off limits.  Also dating relationships are off limits to the extent that you don't involve yourself with someone who is involved.

How do you feel about it? When do you say to yourself "he/she is off limits"?

Or are you a man/woman stealer? LOL