Yes, junks are asian, but so what? if there are millions of european units forced on asian civs, why not force a few asian ones on western civs? like how about caravels, galleons, and other items of that ilk? knights anyone?
besides, can you think of a better name/anim for an early sail powered vessel that isnt stupid?
its about sail power vs. oar power; it makes sense when you play it, but here is the general idea: if you are on the mediterranean or some other heavily coastal body of water, do not bother building junks/sail line. you will have ridiculously slow ships. If you are on the atlantic seaboard, or are otherwise right on a bunch of sea/ocean tiles, build the sail eventually (but not right away... there is really no reason to ever build the junk unless you intend to build it for later upgrades ;p)
in general, just build the galley. it is by far the superior of the two!