Is sexual availability, being available to your partner any time he or she wants, part of your submission?
Yes, it has been.
Why or why not?
Because sexual interaction is pleasurable for both parties. I also find that being expected to please no matter my mood contributes to a D/s dynamic.
Are there limits to this?
Yes. I have never expected to leave work and be there "right now." This would get in the way of affecting my livelihood, which is part of my hard limits.
However, if I am available, I am expected to cater to her wishes.