This may be a fairly unpopular reply but in all situations there are two sides to the coin.
In a number of replies to the OP, the bottom's consent is implied by virtue of being in a darkroom or bar where sex can happen. Or it's expected without it ever being discussed. In the post above @PollaDeMiel actively seeks consent and makes it clear what the bottom is getting into, or in this case what is getting into the bottom.
And to be fair, a bottom should be given to opportunity to consent to someone seeding his arse, where ever he's getting that fuck. Especially if that fuck might deliver him a toxic load.
Another thing about consent is that it can be withdrawn after being given, just as a top can walk away if the 'hole' isn't pleasing in any way.