Today is the first time I've looked for an answer and your right, nobody has a serious answer. I haven't breastfed in six years but I thought that was it. But I also have implants so maybe that? I would bet its hormonal but I'm really hoping someone can post something that would make it better. If it makes you feel any better, you are not alone and I was so upset when someone told me they tasted funny. Doesn't sound sexy to me. I blamed it on the milk.
I first experienced this with my husband, the weird onion type taste, then a lover. I thought it was just because that was how they were built, nothing more. However it recently happened to me, where I was the one with the weird tasting nipples. I am a very hygienic person so it really freaked me out and has made me terribly insecure. I don't have implants so I know it's not that. I tried to think of what the three of us (my husband, my lover and myself) have in common; we all drink A LOT of coffee. I am wondering if that is the culprit? That and I've noticed as of late, I've not been drinking enough water. I don't know if that is what the cause is but I though it worth mentioning.
Try soaking a cotton ball in lemon juice and rub it on your nipples once a week. Works for underarm odor as well. Works for me...... Good luck and hope this helps.....
Onions, like garlic and chives have Sulfur compounds in them. It's what gives them their taste.
Sulfur is the 3rd most abundant in the body and 6th most in breast glands.
You are tasting keratin. It is a sebaceous fluid contained in the Montgomery glands of the areola. This substance works as a lubricant during breastfeeding. That would be its primary purpose in nature. Its secondary purpose would be to keep Daddy from stealing all of Mama's milk from the baby. It doesn't sound like it's working given the amount of persons on the internet seeking to have that question answered.
Still no answer, to all it's 2018 and my wife hasn't been pregnant, still she tastes like onions