Why did Old Testament scholars choose to employ "to know" in a sexual sense?

The word used in the situations you described is (transliterated) yada, which simply means "to know" and is often translated (in different circumstances) find, understand, comprehend and acknowledge.

Considering some of the situations presented in the Old Testament (drunken incest of Lot and his daughters, rape of Dinah, rather suggestive imagery in Song of Solomon, etc), I doubt euphemisms would even be considered necessary; however, Strong's concordance suggests it is used euphemistically (in addition to other senses) in the Hebrew.

I imagine those who translated earlier (KJV) would not have seen reason to translate yada any differently when it referred to sex (they rarely translated words differently solely for the sake of clarity), whereas newer translations (NASB), more for the sake of clarity than anything, use another euphemism (ie. had relations with). I doubt any translators would find a reason to change a euphemism in the original language into something more graphic.

Like many words in English, the verb לדעת in Hebrew has several meanings. One meaning is to have knowledge of. Another meaning is to have sexual relations with. One could argue that the latter is an euphemism for the former, but as a native speaker it does not feel like an euphemism. Hebrew has much fewer words than English, and words are often recycled for differing purposes in differing contexts.

When the Original Testament was translated to English, many decisions had to be made regarding the use of homonyms. A famous mistake was made regarding the קרניים on Moses' head. In Hebrew it is obvious that the word is referring to rays, however in English the word "horns" was used. Would you argue that a horn is a euphemism for ray, or vice versa? I wouldn't. In the same sense "to know" is not used in a sexual sense, but rather the Hebrew word for "to know" is a homonym for "to copulate".