Not sure what should be used here but the current tag (which has 63 posts and counting) is confusing and was made by a random user.
When I first saw this tag I thought it was referring to butts.....

flaccid_bottom sounds like, and after looking into it, looks like, the "recipient" is flaccid. They aren't getting aroused by the prostate stimulation or whatever it is that's supposed to happen through anal.

I really do not see the point in having the tag, though.

Alias -> flaccid

Edit: I did this

the flaccid bottom tag can p well be covered by searching anal_penetration flaccid since its kinda rare that the one that is topping is flaccid.

This sounds like a case for Character-Based-Tagging-System Man! But until he arrives "penetration flaccid" is a good starting spot. The character doing the penetrating probably isn't flaccid.