So, why not the tantric union of two peple in the context of martial relations as they ascend in conciousness toward the realization of their own 'stellar' divinity?( as modeled by the universe). A martial 'roll in the hay', if you will.
"I know not why, but martial men ( and women) are given to love."
Sir Francis Bacon
Sorry Jennifer, I didn't mean to ignore your post.
To put it as delicately as I can, I think the tantric idea of union has unfortunately already been taken advantage of by a number of aikido instructors, and not in a healthy way. Maybe it's just because I've spent more time in the aikido community than I have in other martial communities, so my data pool is skewed, but it seems to be a pattern.
Outside of that, however, I don't know how much of a historical connection there actually is between the original use of the term in China and the later Japanese use. Although I suggested the Japanese might have purposefully not used the Chinese pronounciation in order to distinguish a different meaning of the compound, it is probably just as likely that they had no idea about the previous use of the term, and it's just a coincidence that the two characters were put together again in one compound.
Without knowing who first used the term in Japan, and to what extent the Japanese at the time were aware of obscure Han dynasty Daoist sexual practices, we just don't know. However, given how much obscure Daoist information which was lost in China has been preserved in Japan under different names, it could go either way.
Me, I'm 50/50.