Body-shaming is awful, and people should be called out on it. Bullying men over the size of their penis is inexcusable.
I think if the OP had started from that premise, it would be hard to disagree at all...
BUT she didn't; she started with an example in which a man had been waving his male privilege all over the place ("I'm a man; you must care how attractive you are to me; it's all about my approval of your - and my wife's - appearance.")
OK - still not great to sort of legitimatise body-shaming with the 'small dick' retort, but I think we can all see that the idea was very specifically to attack his macho bullshit in the most symbolic way... so not really actual body-shaming, imo.
And, let's be honest, it's men who've nurtured the whole penis worship thing over the ages. I'd also be willing to bet it's men who use the 'small penis' put down in public more than women. On MN, yes, women do go for it a bit on threads like the one cited - but usually only when prompted by arsehole male behaviour.
I have no excuses for the blatant cruelty of women loudly slagging off ex partners due to penis size though...
This thread does smack a little of the 'ooh, let's catch feminists out, shall we' vibe... Not from the OP, but from others. I'll concede that it's probably true that a bit of double standards creeps in very occasionally, when women think 'enough of this BS' and aren't AS strict at pulling up roughly equivalent behaviour when it flows the other way. Yeah, that might happen I guess. But it's the level of joy that some people get from pointing out the (relatively speaking) small amounts of 'sexist' behaviour flowing the other way that seems so... inappropriate.
Again, not actually you OP, but there are some who positively orgasm every time there's a tiny little hint of inequality displayed from women towards men. There's this idea that, since women have decided to, you know, reject THOUSANDS of years of abuse and bullshit, and to campaign for equality, they'd better fucking make sure those scales are PERFECTLY calibrated, because if there's even the tiniest shift towards women, while we're busy sifting through all the eons of shit and trying to make things equal, we will come down on you like a ton of fucking bricks.
Just my thoughts on the matter.