Amaimon: He wouldn’t react well on it. And the relationship would end one way or another. Either because they played along with it and he would break up with them or they wouldn’t know what was happening and he would get so protective over them that it was a obsession.
Lucifer: Wouldn’t be jealous and just watch what his s/o was doing and watching out if they felt uncomfortable. If not he would just wait for them to come back and ask about the conversation they had.
Mephisto: He’d smirk and quickly make sure that whoever was flirting with them would know that they were taking. Making up would happen very fast and in the form of dominant sex.
Rin: Would be hella insecure about it. He never could understand why they wanted to be with him? Healing over such a thing wouldn’t be easy for him and his s/o would need to do a whole variety of things.
Yukio: Similar to Lucifer he wouldn’t get jealous. He trusts his s/o. If he wouldn’t, he wouldn’t be with them.