Here are my thoughts, for what they are worth.
When you are breastfeeding your baby, you are using your breasts as they were meant to be used, you are providing nutrition for your offspring. This action is not sexual at all, it is maternal. But to have a dh or dp suckling at that milk, well I think that breaks the boundaries between maternal and erotic. In my mind there is something pure and even innocent about a mother breastfeeding her child, to turn that into a sexual thing is tainting that image and it is tainting breastfeeding.
Not saying that those who do this are perverts, but personally I find it distasteful (scuse the pun!). I couldn't even let my dh touch my breasts when I as bf, they had lost their sexuality to me, they were for the baby and that was that! It was only after I'd stopped that I allowed him near them again! I understand that many men find it fascinating and during sex they might want to take a mouthful, I just find it strange that the mother would be turned on by this. And as for feeding your dh/dp at the same time as your baby, well that's turning a beautiful experience into something that it just should not be.
It's so sad when as women, we cannot do anything without it being perverted in some way, from giving birth to feeding, there will be someone who will want to sexualise it. Just adds to the notion that women are sexual objects, nothing more.