> and just translated "hot" verbatim - as in the adjective indicating temperature

Well, "горячий" does have figurative meaning "passionate" (and by transfer, "sexy") in Russian just as it has in English. Heck, English is even worse in this regard: "heated argument", seriously? Not only an argument doesn't have a temperature, you can't change it either (since it does not exist)! Yet the phrase exists just fine, and it translates as "hot argument" to Russian, no problem.

No comments on "MILF" though. But I wouldn't be surprised if it actually entered the (youth/Internet) slang as-is: many other English words did as well.