The "quotation" as usually reported has absolutely nothing to do with Socrates. The website you consulted to trace it to Aristophanes' Clouds -- that's a fairly popular attempt to rationalise the "quotation", as coming from the contest between the "Right Argument" and "Wrong Argument" (or, we might paraphrase, "traditional aristocratic ethics" and "new-fangled sophistic ethics"), spoken by "Right".

As you've no doubt realised, it doesn't actually appear in that play either. At best it's fair to say that the same sentiment is embodied by "Right" in that scene. I've no idea which modern writer invented this popular "quotation" that isn't a quotation, and I'm not really interested; but you're not asking about that, so let's drop that matter.

However, the specific bit you're asking about -- crossing legs -- does have echoes in the Aristophanes passage. Here's a less archaic translation (tr. Ian Johnston):

. . . Then, when they went outside,
walking the streets to the music master’s house,
groups of youngsters from the same part of town
went in straight lines and never wore a cloak,
not even when the snow fell thick as flour.
There he taught them to sing with thighs apart. ...

. . . At the trainer’s house,
when the boys sat down, they had to keep
their thighs stretched out, so they would not expose
a thing which might excite erotic torments
in those looking on. And when they stood up,
they smoothed the sand, being careful not to leave
imprints of their manhood there for lovers. ...

When he was eating, he would not just grab
a radish head, or take from older men
some dill or parsley, or eat dainty food.
He wasn’t allowed to giggle, or sit there
with his legs crossed.

There are three potential echoes here.

• 966 "he taught them to sing with thighs apart" (ᾆσμ᾽ ἐδίδασκεν τὼ μηρὼ μὴ ξυνέχοντας). The ancient scholia on the play report that this referred to preventing children from secretly masturbating by rubbing their thighs together. Of course it's always possible that this is just the lurid imagination of some ancient scholar, but it's a good fit with Aristophanes' tone.

• 973-4 "when the boys sat down, they had to keep / their thighs stretched out" (καθίζοντας τὸν μηρὸν ἔδει προβαλέσθαι / τοὺς παῖδας). Aristophanes himself explains this one: the idea is to prevent the boys from exposing too much and thereby arousing too much interest in onlookers.

• 983 "or sit there / with his legs crossed" (οὐδ᾽ ἴσχειν τὼ πόδ᾽ ἐναλλάξ). Neither Aristophanes nor the scholia give a direct clue here. However, the scholia do report that radish-heads, etc., were considered to be forms of aphrodisiacs: the thrust of these lines, then, is that the young boys are supposed to be avoiding things that might excite them, or (as in the previous passage) teasing/titillating behaviour that might get onlookers excited.

So you're certainly right that in the Aristophanes passage the intent is to avoid sexual entanglements, but not quite so directly as by physically preventing penetration: rather, the speaker -- a satire of an old-fashioned aristocrat -- regards the youths' untamed behaviour as titillating.

(Incidentally, this is aligned exactly opposite to where Socrates would fit: Socrates is, as it happens, a character in the Clouds, but he represents the side of "new-fangled sophistic ethics", i.e. the opposing side in the argument that this passage comes from. The opposing speaker in the argument, as it happens, is largely uninterested in sex: the above speaker's preoccupation with titillation is, again, a satire on old-fashioned individuals and their supposed obsession with sex.)