I think the mating is a lot more effective as a treatment than the toys/medications are, which is why Ise uses them. The omega cat girl (who Kumo helped previously) seems to be just fine after being "treated" by Ise, while Kin, on the other hand, seems to be having a hard time with his heat even after the toys and meds - constantly feeling horny, dizzy, feverish, etc. There actually isn't a reason for Ise not to help him, as a doctor. The high school girl he treated, theoretically, would be just as innocent as Kin is, so why would it be fine to treat (mate with) her, but not him? I think the answer is that he's trying to protect himself just as much as Kin. He knows they love each other but, as a doc, he also knows he can't bond with Kin without putting his patience in pain - and that "infidelity" would hurt both, if they were to be together. Thus, he tells Kin to find someone who could "properly" love him. At least, that's my theory.
I think Ise is in love with Kin too, but since his job requires mating with many omegas, he can't really bond with Kin; also, if he were to sleep with him, he probably would bond/not want to mate with anyone else, reason why he refuses and tells Kin to find someone who will be able to be with him and only him forever. Kinda sad. I like Kumo and Kin, but Ise and Kin are cute too ╥﹏╥