Vision wasn’t “posing as human,” he is a unique hybrid organic being known in the MCU (and the comics) as a Synthezoid. So he actually can have sex.
Recall in Age of Ultron that Helen Cho used her Regeneration Cradle to actually print organic human-like tissue, and that tissue was then somehow bound by Vibranium at the atomic level. So he is essentially entirely (or mostly) human like, but with all his cells coated in Vibranium.
Therefore he is physically no different whether he changes his appearance or not.
And there is ample evidence that Vision is sexually active in the MCU starting sometime before the events of Infinity War.
It is interesting to consider if he could actually impregnate a human woman, but I would think that logically even if he did, that offspring wouldn’t be a Synthezoid. But who knows what’s possible with Vibranium in the MCU, as it seems to be the magical deus ex machina to end all magical deus ex machinas.
You may be wondering if Vision wasn’t just basically a sentient ‘play thing’ for Wanda. But this theory has no legs after the events of WandaVision, where our favorite oddball superhero couple sleep together, and Wanda soon becomes pregnant. (Of course this was Wanda’s work via her magic, but it was subconscious and she wouldn’t have believed she could become pregnant if she wasn’t having, um, ‘traditional’ sex with her new husband.)
And of course we can all wonder what thoughts ran through Wanda’s head that morning after being the first human to bed a Synthezoid.
Insert Vibranium boner jokes in the comments. 🤷♂️
Thanks for reading: While MCU Vision was posing as human, did he have the ability to have sex?, Sorry, my English is bad:)