Gijinka / Moe Anthropomorphism - Human Rainbow Dash Pec Flex

Who needs gainaxing when you have swole pec dance?

If it's possible for guys if they build and train enough muscle, then there's no doubt a woman who builds and trains her muscles could do it too.
It's just that, like with other muscles, flexing is mostly meant for showing off your body, and since women already have breasts for that, flexing their pecs is kinda pointless.
At least, that's my best guess for why neither you nor me have ever actually seen a female pec flex. Though the freakiness of the mere sight of this pic might be a more likely reason why women don't flex their pecs.