The start slave has a special line of 4 events. For the first 3 events, the player chooses from among the 3 types of options. In the forth event there is a sex scene with the dialogue you enjoyed. Complete map of start slave quest: https://itch.io/post/543902
The choices you made during those events determined the results in the 4th event. Nothing else has any influence and them having the "Dominant" trait was effectively a coincidence. That quest occurs only once per progress and has very little relation to the rest of the game. The game is not designed around the slaves being mechanically dominant and the dominant trait has very little impact on their sexual actions (google the wiki for traits in Strive for Power for specifics).
The best that you will get in vanilla v1.0a is having them be the giver for all actions in the sex interactions and let them lead the sex. Unfortunately, the dialogue will still make them sound rather submissive as the game does not particularly differentiate the slaves based on their limited "personality".
If you have a copy of v0.5.25 (it will not be compatible with v1.0a), then I would recommend trying the Chastity Belt mod (https://itch.io/t/554106/chastity-belt-mod-v122), as it adds mechanics that allow slaves to be mechanically dominant. It may not be all that you hope for, but it will be the closest you get without making your own mod.