I'm not convinced, but I am sure that whether or not the Hebrew word refers to the Behemoth's sexual parts is not what I heard a preacher refer to as "the root of the matter", in other words, an essential part of the gospel. Incidentally, I don't think Ruth would be lying intimately with Boaz, as they weren't married at that stage.
Sexual euphemisms are used in excess in the old testament. And as noted before, the Vulgate very plainly just translates this as "testiculorum".
I'm not sure why you're fighting this one.
Why do you think it's important for Ruth to lay at boas' feet? Because it's not actually talking about his feet.
Isaiah 7:20
Ezekiel 16:25
1 kings 12:10
Are more examples of this.
Isaiah 7:20 NIV
[20] In that day the Lord will use a razor hired from beyond the Euphrates River—the king of Assyria—to shave your heads and private parts, and to cut off your beards also.
Isaiah 7:20 ESV
[20] In that day the Lord will shave with a razor that is hired beyond the River—with the king of Assyria—the head and the hair of the feet, and it will sweep away the beard also.
Here it is again. Compare the ESV with the NIV translations. Feet is interchanged with private parts.
Ruth 3:3-9 NIV
[3] Wash, put on perfume, and get dressed in your best clothes. Then go down to the threshing floor, but don’t let him know you are there until he has finished eating and drinking. [4] When he lies down, note the place where he is lying. Then go and uncover his feet and lie down. He will tell you what to do.” [5] “I will do whatever you say,” Ruth answered. [6] So she went down to the threshing floor and did everything her mother-in-law told her to do. [7] When Boaz had finished eating and drinking and was in good spirits, he went over to lie down at the far end of the grain pile. Ruth approached quietly, uncovered his feet and lay down. [8] In the middle of the night something startled the man; he turned—and there was a woman lying at his feet! [9] “Who are you?” he asked. “I am your servant Ruth,” she said. “Spread the corner of your garment over me, since you are a guardian-redeemer of our family.”
Sorry to break the news to everyone. But it's not talking about feet.
And that's exactly what is happening with behemoth. It's using the same style of writing, common in the old testament where rather than outright saying it, it's using words of double meaning in a clever linguistic style. And so some translators such as King James, recognize this and use the word "stones" or "testiculorum" while other more word-for-word translations like NASB just leave it as "thighs".