I wouldn't say breastmilk rots teeth, but I think that some people are more susceptible to strep mutans (the bacteria that causes dental caries) and for those people, having anything in your mouth all night is going to cause caries. My DS1 had his first cavity at 9 months, and at that point he'd never had sugar, a bottle, cow's milk, juice, candy, antibiotics and I didn't take abx during pregnancy and we both took COL--but he was all night nurser. He also breathes through his mouth at night, another thing that can promote caries.
Anyway, his front tooth rotted away by 16 mo. Our dentist took it out under conscious sedation. That was at my DH's insistence, and we both regret it--I wish we'd done the GA, but I guess the good news was it only took about 10 minutes, including capping the nearby tooth. So he has a cap. At that time he had a tooth pulled, one capped, one cavity filled and one started, but not big enough to fill. We got fluoride sealant and went back every 6 months and started using pea-sized amounts of Tom's fluoride toothpaste every other night, w/ Spry xlylitol gel every day. It stopped the decay cold and we haven't had a cavity since (knock on wood). He's 5 now and his diet is awful compared to then, so IMO it's the fluoride that made the difference. (He continued to night nurse until 2 or so).
At that first appointment, his teeth were soft and horrible, now they're great. DH and I are convinced night nursing was THE problem. He had classic "bottle mouth rot" but without the bottle. However, I don't think night nursing causes cavities for everyone. My second is still night nursing at 2 years and has great, strong teeth. But he isn't a comfort nurser, nor does he nurse as much as my first did.
Anyway, sorry for the ramble, hope some of it helped. Try and join the Yahoo group "Very Young Kids Teeth" if it's still around. I got a ton of helpful info there.
ETA Try getting some xylitol products and using them. We still use Spry because my boys love the taste. Xylitol reduces the growth of strep mutans. When he was 2-ish he started chewing Spry gum, esp after meals. They also sell xylitol wipes (I got mine off eBay) but I never got much use out of them.
Also, I hope you don't beat yourself up over this. I felt so much mama guilt, especially since my LLL group and many others would vehemently defend night nursing and suggest that if I only went Traditional Foods (which I did) or took fish oil (which I had already been doing) my son wouldn't have had cavities. Not true--for whatever reason, he was just susceptible to the strep mutans bacteria. Don't feel bad, and don't worry because teeth problems can be corrected.
