One point of clarification--it's not "feet" exactly, it's "legs". Hebrew does not have distinct words for "foot" and "leg", nor between "hand" and "arm" (there's a word for "forearm", so you can make the distinction, but there's nothing wrong with calling your whole arm the same word as "hand"). The word רגל regel (often in the dual plural רגליים/רגלים raglayim. The standard plural is רגלים regalim, but I believe that's only used to mean "pilgrimages", and body parts that come in pairs are pretty much always in dual plurals anyway even if the regular plural were an option) in a biblical contexts sometimes means feet, and sometimes is legs, and sometimes is a euphemism for the genitals. But knowledge that it also means "legs" makes the euphemism somewhat more comprehensible.

Even in the Hebrew Bible, there's no need to assume that all or even most uses or this word are euphemisms. Some of them are, but not all of them are. Foot washing is a known practice of many societies, and when most transportation on foot is dominant and without modern footwear washing feet makes a lot of sense.

However, there are occasions where "washing feet" could be a euphemism in the Hebrew bible. In Kings David tells Uriah to "go home and wash his feet" when he is telling him to go home and have sex with his wife (in order to conceal the affair between his wife and David). But it also could be that "wash his feet" is simply a way of telling him to go home and get comfortable, and that is the euphemism. To complicate things more there are later Jewish texts which discuss washing of feet as something wives should do for their husbands, which sounds like it's a euphemism but probably isn't. That's much later, of course, but still is an interesting way to frame the intersection of this being sexual in nature or not. But washing of feet is definitely not always a euphemism, sometimes it's used when it seems feet are literally being washed.

So "washing of feet" can be a euphemism, if context suggests that it is, but it needn't be. And that's just in the Hebrew bible, which is where the "feet are a euphemism" thing comes from. I can't speak for whether that would make any sense in Greek, but certainly if it were in the Hebrew bible it'd be dubious to suggest it's a euphemism, so all the more so in another language it's not terribly likely.

I don't know whether this is sourced enough to be allowable in here, but I don't think those passages in the NT are referring to genitals. In the Hebrew Bible, there are several passages, like the one you brought, where it makes no sense if it's about actual feet. For example, Naomi tells Ruth to "mark the place where he shall lie, and you shall go in, and uncover his feet, and lay you down; and he will tell you what you shall do." (3:4).

None of the passages in the NT are in obviously genital-related contexts. In addition, we know from other Jewish sources from around the time of the Gospels of traditions of people washing each other's feet. Also, the gospels were written in Greek, so there's no particular reason for that kind of euphemism to carry over into a whole different language.