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.
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.