Could request the Final Word and ask for clear communication. Maybe something like...
1) I want to be in a polyship with my BF (him) and with my GF (you). This includes sex expression.
2) I will not promise permanent sexual exclusivity to just you. This is a limit with me.
3) If permanent sexual exclusivity is a NEED for you in your romances, I cannot meet your need as such. If you require a short period of time of abstaining to adjust, I could be willing to entertain the thought, depending. How long is the time you require? What would you expect of me in that time? How can I support you in that time?
4) If sexual exclusivity is a WANT for you in your romances, and one you can let go of then perhaps this can work between us that way. How can I support you in being willing to let go of the want?
5) We have had this conversation before, yet we do not arrive at a conclusion together. I have a new limit. I wish to know your final word now, if you know your willingness to continue in polyship with me. If not sure, could you know by the end of the (week? month?). Otherwise I have to move forward without taking your information on board. I need to relieve my suffering here in limbo land. I do not wish to continue to suffer.
6) Please clarify: Is sexual exclusivity a want or a need for you? I can respect both your want and your need, but I need a clarify so I can deal with my own self and my own life and my own wants, needs, and limits. Are we compatible enough so we can be in agreement and in harmonious romantic relationship or not? Is this about jealousy management? Or relationship shape preferences? Could we better as friends?
Try to sort it out. Not all dating partners are going to be a runner. Sort this one out, give it a fair shake, and if it is not lining up, accept it is not a runner.
Could accept you both seek different models, have different wants, needs, limits. Could end the romanceship shared and shift gears into friendship shared instead.