There is no romance with mistress, not anything that isnt one sided. Mistress is training, but especially conditioning the new sex slave. Thats her job. Hence teaching her how to be with both sexes, but obviously the with a man part was going to take more work. Thats why mistress got her used to sex as a source of pleasure and comfort, then started acclimatizing her to penetration regularly before she started taking her around town to be around men, and you can bet that the area she took her was filed with the better crop of male specimens. You need to accompany mistress was a pretext to get the main character around flesh and blood men now that her defended had been completely obliterated.
That doesnt mean that the main character wont still have relations with Mistrsss, or other women, but Remember that this kind of role is what the OC always wanted deep down.