Why do tops in porno's first get their holes rim before they shag the bottom

Fair question, however leading it may be, suggesting:

a. Top should be rimming bottom, prepping him to get fucked
b. bottom doing more than his share while Top just needs to get hard
c. Top getting rimmed just before fucking bottom up the ass is selfish, especially in prep of fucking

I reckon we're conditioned to conceptualize porn in a rather generic, predictable contexts, internally and externally. As a TOP, I CAN say that when getting great head, my bottoms intuitively work their way around, burying their mouths and noses in my ass...

By then, I'm often so revved that I'm not 'thinkin' about anything other than throwin' that boy down and fuckin' him stupid...no p/c bullshit...it's about THAT moment, and it's up to me, HIS TOP, to KNOW when that is...and have the balls to see it through...

Or, maybe those bottoms just can't help themselves and dive-in when given the chance, a demonstration of his desire to service his TOP...THIS is where my mind goes, given the meaty, very fuckable asses that a lot of these hung TOPS (in porn)...

Curious much?! Ask me anything...sexuality and sex-related topics preferred.