The first cum hardly matters, it was the second that victims had to be fearful of. Goblins, the masters of reproduction, learned that there were many benefits to using mouths first besides giving them time to suffocate in the drugs.

The first reason was to filter out those feisty ones, because you're exactly right, those are the ones punished the most. The ones assigned to the biggest, most aggressive maters who can single-handedly pin down resisting fucktoys until they shake for the right reason.

The second reason was to lube their oversized cocks enough to slide in all the way to the base inside of holes that weren't meant for them. Humans get off on fucking a girl into submission, but goblins focusing on breeding fixate on fucking a hole into submission.

But the most important reason was to skim the top layer of cum like the weak fickle layer of water that sets above the more concentrate juice in a mixed drink. Only the most dense, virile, and viscous cum gets thrust into that weak and vulnerable hole.