[Marsh.]

Except Dexter was using the analogy that Callum insinuated he was.

Dexter said "spank my arse and stick it in me" if that's not a reference to being anally penetrated then I don't know what is.

So, as Callum said, that's either referencing having control over someone metaphorically or literally shagging them. A way of saying you are superior.

Also Dexter seemed under the impression that it was said weeks ago, not on that night.

The fact Dexter used the same phrase on that night in reference to himself just shows that Dexter uses that phrase.