My H wanted the same thing - said he had "low self control at the moment." I just calmly told him that he and I will not have a romantic or sexual relationship whilst he is involved with someone else.
Just state your boundary and stick with it.
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus