Loose is a Very Wise Man

>> boobs = circle circle dot dot


♪I got my cootie shot♪

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Eyes. I see what you're getting at, but people usually have to draw something else to emphasize what body part it is, while a penis is something that's rather recognizable because of how the shapes stand out, and how those specific shapes in conjunction with each other don't have many other things that we can fit it to off the top of our heads. If you're doing two circles and, well I won't go into detail and risk an infraction, but it's actually less work than breast, and I think even drawing one in a detailed sketch would be easier since you don't have to place it on a body. It can literally be floating and people would know what it was. You can try that with breast if you're goon enough to detail the nipples to actually look like nipples, otherwise it's floating circles and needs a torso to accompany it in order to provide the full image.

Circles within circles are everywhere, but phallic shaped objects with 2 circles aren't as commonly seen in everyday life (aside from on your body if you're a guy). It's just easier for your mind to relate simple shapes to a penis than it is to relate simple shapes to breasts.