RE Vision’s down under: In the instances in the comics and the MCU where Vision has appeared naked, he’s shown to not have any genetalia/reproductive organs (which is likely why he’s allowed to show full frontal at all), and in either universe there’s not really any logical reason for him to be created with them. But, as you said, he could probably make whichever genitals he’d like if he felt like it, and even if he can’t, he’s getting by, because you don’t need a penis to have sex. Be creative.

I’ll take this opportunity to clarify something about Vision:

Avengers vol. 1 #57

Vision’s body was not originally robotic. John Byrne introduced that retcon because he didn’t think Vision should count as a person at all. As he was originally written, his body was entirely that of a human male, just synthetically made. He could even bruise and bleed:

Avengers vol. 1 #132

So much like the idea that Wanda never remembered her kids before Jan let it slip in Disassembled, this was a retcon that directly contradicted previous continuity, instead of just revising our perception of it, so it’s not a clear answer for comics Vision, especially since he’s had several different bodies.