As I grow older, it is known to be a brutal fact that most nice girls are already taken (or at least it is probabilitily correct, and believed so). Usually they would have high school sweetheart boyfriend or boyfriends since god knows when. It would easily cross my mind to go for girls who are already engaged in a relationship instead.
For me, I found it extremely uncomfortable to go for girls who already had a boyfriend. First of all, I feel quite uncomfortable with the idea (thus came the stupid principal). It just doesn't feel good; it is kinda gloomy. Why would I want a girl who like/love her boyfriend? A very strange question in fact, which shouldn't be asked with a thousand reasons. We human are not perfect, and are never meant to be.
Second of all, girl who is engaged cannot admit openly they like you (not that single girl do that very openly and often as well). You shall always end up in guessing state, where did she actually like you or you misunderstood the whole thing. Though there are feelings and body language, but we need some sort of reassurance as well. They can't tell you they actually like/love you, but neither do they say they don't like you. So, you are left hanging in mid-air, WEEEE!
Lets say you are a lucky bastard who manages to capture her heart and she truthfully admitted that she like/love you as well. Then came to next big challenge: Getting her to leave her boyfriend so that both of you can be together. Personally, I feel it is kinda difficult for a girl to leave their boyfriend (unless they are being treated like shit and needed to be rescued). Theoretically, you can only be in one relationship at a time. Thus, she can't commit to you before she left her boyfriend. Yet, she might be reluctant to leave him as they had been together for so long, and he is not that bad after all (and can be sweet at times). Besides, being with you is actually a risk and there is no official commitment yet. In one hand she had something concrete with commitment, on the other she had you with nothing. She might enjoy your company and might even like you a little bit, but being with you is a whole different ball game. The fear of lost is greater than the joy of gain.
So, what it takes to make this kind of relationship successful? Probably you need to be very sure she is the one, and go all the way (I mean ALL THE WAY, THE WHOLE MILE, NO HOLDING BACK), in a big way. You need to take on a very proactive role and be very aggressive. Once you are sure, you no longer ask, you just do it. Is like you don't ask for a kiss, you just kiss, and action should work much better than words in this case. You need to have courage and be a man of action with 100% faith and no return. I hope I am not giving a very bad advice here.
Basically I have no idea how it should be, but I believe the above way is the way. Either it is done, or it isn't at all. I always felt it is wrong to do such thing, but someone once told me, "You only live once. There is nothing wrong is chasing what you love". Perhaps there is some truth in it, and after all, it took both hands to clap. Sometimes I also hope that I am a man of courage and action, but too bad I am a man of words instead (look at the number of words I had typed). If only I know...