With my former Master he told me fairly early on that he had a long term sub which, after the initial shock, I didn't have a problem with as he allowed me to play with other Doms as long as I wasn't marked in any way. Over time though (and after he had released "No. 1" and I became his partner in the vanilla world) I felt less comfortable playing with others so when he asked me to be exclusive to him I was happy to agree. He was clear that this exclusivity would not apply to him BUT was also clear that he would not release me simply for a 'better model'. Also he was extremely discreet so unless I wanted to know if he was Doming someone else - and occasionally I did ask - it didn't intrude on our relationship in any negative way. My current Master has made it clear that monogamy is crucial unless He wishes me to play under His instruction and I am very happy with this. So, horses for courses!