Thank you FTjr. This subject is for many, taboo; therefore, it was very brave for you to come forward and admit your pleasure.
Whilst your post made sense, something is still not clear to me. You state that you enjoy watching the submissive derive pleasure from the act and in the next paragraph suggest that it may be a "hard limit" for her and would prove her ultimate commitment to submit to you.
If, indeed, it's a hard limit, why do think she'd be enjoying the act? Could it be that knowing she's not enjoying it and doing it despite how much she's repulsed by it, is exciting to you, because she's doing it for you?