Kosher is complicated.
While we're at it, wine is different. Very different.
There's inherently nothing non-kosher about wine, any more than any soft drink. But. If the open bottle is in the possession of a non-jew there was a fear that some of the wine would be poured from the bottle to appease/honour an idol (or, later, to use in the sacrament). That would clearly pollute the whole bottle, it is termed "Yayin nesech" (lit: poured wine).
What about an open bottle of wine that MIGHT JUST have been used - or thought about being used by one of these HEATHENS? (coz thinking about it is obv just as bad) The Rabbis thought about this and decided that it should be treated as if it were "Yayin nesech". Forbidden.
Just in case you missed the implication, a "kosher" bottle of wine becomes non-kosher if it is open and handled by a non-jew, some would say, if it is alone in the same room as a non jew, a unique category.
That's perfectly clear then. Only, not everyone is necessarily an idolator or a Catholic. So, why does this apply so universally? There, we have two explanations. The first is simple. You Can't Be Too Careful, they might be secret idolators. The second is more curious. The rabbis recognised that drinking with people would lead to stuff, and hence intermarriage, so said "stuff it, no way are you allowed drink with non jews"
