I find that the argument for NOT 'going after someone already in a relationship' is a very strong one. I agree with it totally! So I dont need to add any further points to this. However, having had this very thing happen to me recently whilst dating a girl for 2 years, I must say it can be very emotionally draining on everyone involved. All I can say is what ever position you are in, whether it be the pursuer, the pursued or the bystander (ie bf or gf who is already in the relationship), the best thing to do is to look to God through it all. If you have your hope in God, you will no longer look at your situation as a "your" personal problem, but know that God has something planned and it will be good!
God has a way of sorting these things out if you let Him, and it will reflect in your strength of character. As far as searching for partners is concerned, I would rather find someone more in love with God than with themselves or with others. If someone did end up letting you down, wouldnt that show where their heart is anyway?
Finally, if you are the pursuer, well... firstly, you have no assurances that you are the better man (or woman) for them, especially if you are pursuing them and not God! If you are meant to be with them, God is a better judge of that and He will do it His way. Better for you to look to Him and rely on Him and to not pursue something which will only show that your heart is in the wrong place. Thats what I think anyway
In His Love,
JW.