I know everyone's relationship is different, but I have to disagree. If the OP and her primary have decided together to let a secondary into their relationship, she has every right to say that birth control will be used. Before anyone slams me let me point out how she said it.
While all of it is plausible, I did state that for my comfort that birth control will still be used.
She didn't say you are not allowed, she stated for her comfort. Now if the others choose to ignore her, then she has the option of leaving, but she has merely pointed out what she needs for her comfort.
I am the same way and everyone is in agreement. My husband knows that I will not accept another baby into the family. If he chooses to not use protection and gets his girlfriend pregnant, I leave, end of story. It is something I cannot live with and I choose early on to express that anyone involves knows how I feel. She has a right to express her feelings, especially early on, so that all the cards are on the table.
However, yes it could have been handled a bit more delicate. But you did the right thing now holding it in.
Purely my opinion and feelings on the situate. As I said in the beginning not everyone will agree.