What does Marco's answer to the Khan about women and wine mean?

The question he was asked was a trap, any simple answer he gave would get him in trouble.

If he answered anywhere but Mongolia then he would offend his hosts. But answering Mongolia would be seen as spineless flattery.

Instead he deflected the question and answered by saying "The true sweetness of wine is one flavor". This means that while there may be many different flavors of wine in the world, they all taste good and all get you intoxicated.

In other words he finds women everywhere desirable.

It was an answer cleverly avoiding the trap in the question and by doing so continuing to keep the Khan's interest.

To expand some more - the phrase is ambiguous and fairly meaningless in of itself. It sounds all mystic and wise but actually has no real meaning. This in itself is deliberate. It allows people to read whatever meaning they want into it. As soon as someone in the court laughs then everyone else has to laugh along or look like they didn't understand the joke.

If it does exist as an Italian saying (and I've seen no evidence to say that it does) then with reference to that culture it may be possible to assign more concrete meaning to it.

The way I interpret it, is that while there are a lot of different flavours of wine, ultimately what we all enjoy from wine is the alcohol, which is the "one flavour" he's referring to.

The same can be said for women. While there are women from different ethnic backgrounds around the world, what we men enjoy ultimately, is the woman herself, no matter where from. The "flavor" part can be left to interpretation, whether he was just referring to women in general or the taste of their parts.