AFAIK the 'if it walks like a duck' saying is based on Occam's Razor, which suggests that if something seems obvious it probably is, so you shouldn't jump to conclusions - the phrase often used to exemplify it is 'if you hear hoofbeats, think horses not zebras', which is taught in medicine to tell doctors not to make unnecessary assumptions when looking at symptoms. It really doesn't apply to false penises, which require numerous assumptions to be made before anyone could assume them to be real.
The 'quacking penis' line was obviously a joke, based on what an already an attempt at humour, so I can't see how it could be taken as disrespectful. There is a huge difference between that and a 'LOL' at someone's post, or an 'I'm ignoring you' comment that basically cancels someone's contributions to a debate.