When you´re in a session with a sub then it can happen that things get hot, you get aroused and feelings of lust are no stranger at these moments. So how do you react to that? As a professional dom/domme it´s probably not a good idea to give in to such temptations during the session, but afterwards you could explore such feelings and see where it goes. Though it might not be a good idea to keep this sub as a client, as the lines between business and pleasure should remain even if they can be a bit vague at times.
During a session a client of mine, a transwoman, got an erection. She was very ashamed of that and couldn´t stop apologizing. She takes hormones but I guess when things get too hot it´s not avoidable to get turned on. I don´t see it as something you should apologize for, in a way I take it as a compliment because it means that I´m doing something right. It´s not that I´m completely cold to the things I do, seeing someone bound in rubber makes me wet just the same as most latex lovers but it doesn´t mean that I break my own rules.
But maybe sometimes things can get out of control, then it´s better to end the session. As tempting as it might be to have sex at those moments, sometimes it´s better to walk away than to give in. If the feelings remain and the other has similar feelings then you can always explore those in your private time. It´s not impossible or unthinkable that a relationship is impossible between a dom/domme and a sub, but a relationship with a client is probably not a good idea. Still you can move from dom/domme and client to a more personal relationship.
I can definately advise you to talk with your dom/domme or client if such situations might occur, in the end it´s just human nature to get turned on by something or someone you like. If you have a good connection with the other then such a conversation shouldn´t feel awkward or unpleasant, just be honest how you feel and who knows maybe the feelings are mutual. There is nothing wrong with a sub licking your boots, but what if he or she could be licking something else...