I too am outraged to hear how your meeting with the Bishop went. I would also be looking to move up the chain until you are satisfied. This is completely wrong for them to make YOU the issue.
Just out of curiosity, I asked my DH about this. He has had struggles with porn in the past. He hasn't had a real addiction, but he has had a hard time for most of his life knowing how to act when he sees a bit of flesh, and how to keep his thoughts under control. He too served a mission in South America, and did have a hard time when the young attractive ward members would openly nurse, even though he saw his mom nursing all the time. After discussing your situation fully, he admitted that yes, seeing a bit of breast while a mom is nursing could start a chain reaction of thoughts. It wouldn't make him go look for porn, but it could start that chain reaction. HOWEVER (before I get a bunch of flaming posts about how horrible he is!
), he is ADAMANT that it would be HIS problem and not yours. Never in his wildest dreams would he ask you to stop, or ask someone else to ask you to stop. He fully believes and understands that breasts are for nourishing babies, and believes you have every right to nurse in whatever way is best for your baby. He is a huge supporter of NIP. He said over and over, "My issue is MY problem, not the nursing mom's problem. She is doing nothing wrong. I am responsible for my own thoughts, not her." He has over the years learned how to control those thoughts in himself, and how to focus on something else.
So, yes, a bit of flesh might make it difficult for someone sincerely attempting to recover from that addiction to keep his (or even her) thoughts on track, but this is something they NEED to learn how to do in order to be successful anyway. He took the smoking comparison and said, "It isn't like she's inviting him to join her! It's like a smoker who's on his way to work and passes a few people smoking on the way. They aren't asking him to join, they're just doing their own thing. But he needs to learn how to keep himself from being tempted by the sight."
So, in reality, you're providing an opportunity for his spiritual growth!
Anyway, we both support you and hope that you can find peace and resolution.