I presume you're only talking about pineapple juice which is rather acidic and not good for your teeth (particularly when consumed frequently and in excess), however it's not any more acidic than most other fruit juices. Fresh pineapple, along with many other fresh fruits are obviously fine for your teeth (except perhaps if you have too many citrus fruits etc).
I presume the enzyme in Pineapple you're referring to is bromelain but regardless, I'd love to know how the enzyme could have any effect on enamel whatsoever. I presume the effect is indirect (i.e. the enzyme affects some other substance or process/chemical reaction, which in turn damages/demineralises the enamel) but I can't think what it would be?
Anyway, the advice you were given (about drinking milk/water/waiting) sounds correct but only due to the acid content of pineapple juice (not any enzymes as far as I'm aware) and the same applies for every other fruit juice too. Indeed, the advice pretty much applies to eating or drinking anything if it's right before brushing, ideally you should wait but if not at least drink some water (or rinse with it) before brushing.
As for taste, based on numerous girls I've spoken to (WGs and non-WGs) some can detect things in your diet (usually strong food and drink) like if you've recently had quite a lot coffee. Strong and spicy foods (e.g. curries) can also have a negative effect on the taste. As such, it doesn't surprise me at all that the WG noticed a taste difference after quitting smoking. Also they do say that cum can taste sweet (as well as slightly salty), particularly if you eat fruit and particularly pineapple which can apparently give a hint of "pineapple taste". I'm sure it depends on the girl, their sense of taste etc but I certainly wouldn't rule it out as a myth, at least not for many girls.
I'm not too sure, just running off what my enzymes professor told me, I sucked at organic enzymes though.
However the whole diet thing has little basis when you look at biology, your testicles are not part of the waste system. Your piss and shit will change, as that is part of the waste system, but your spunk isn't in those systems. They are separate from waste removal.
A hint can be left from remnants of urine remaining in the pipe, but most won't pick that up. And actually a sweet taste would be quite concerning as it would hint at diabetes.
The told tales are mostly caused by convincing yourself of a fact. Your brain can over ride what your senses tell you is true.
Spunk is naturally slightly bitter (despite the fact that after water it is mostly carbs, despite the tales it's actually very low in protein). By lowering tea, coffee, alcohol, and adding more fruit and vegetables, you're increasing your water content which can lower the bitter hint.
But everything is anecdotal, there are no proven studies showing how diet affects taste, so people who have an agenda to make you eat more of x will tell you it improves things that they have no scientific basis for.
