I agree with the people who say that a boyfriend or girlfriend can't really be "stolen"...
However, with that said...
I've been the "victim" of this sort of thing. Obviously enough, it sucks really bad.
I wouldn't do this to someone, and I especially wouldn't get with someone who was involved with a friend of mine. I wouldn't even get with a friends ex. I can understand the whole "well sometimes you just love who you love" thing, but for me, I just can't and I can't really see how other people can live with themselves when they do it if it's obvious their friend isn't really okay with things being as they are. I guess if everyone is cool with it, then I wouldn't be judging anyone. People should do what makes them happy, but IMO the bond of friendship is sacred and you don't fuck with that shit. I can't even manifest any sort of attraction for a person once I found out they were involved with a friend.
And I agree again in the case of home wreckers, the husband or wife who is doing the cheating or allowing themselves to get emotionally entangled with someone else is really who is to blame, though the "other woman/man" isn't innocent either if they know the other persons relationship situation. Still, in the end the person who made the commitment should be the one saying "no get the hell away, I'm taken/married/whatever".
I'm fine with the fact that people have differing views on the morality in these situations, but mine are pretty set. I wouldn't want to be involved in a relationship with someone who thought different, or the possibility would always linger in the back of my mind. For that matter I wouldn't want to have close friends who thought much different, especially after experiencing this first hand. I simply wouldn't trust them otherwise.