What’s with yaois and armpit fetish? Genuine question

It's not yaoi thing lmao.

My guess it's because it's a hot sweaty place that you usually don't see
But it's not just yaoi I've seen it in straight stuff too

im sure its not just yaoi but yes and feet too D::

It's a relatively common kink. Any body part tbh; feet, thighs, hands, back, nape, etc.