I'm sorry for criticizing the girl, but it's not her business if he decides to go out with whoever he wants. She also shows no consideration for the other person involved, disregarding his and everyone else's feelings. She's disrespecting their relationship and seems to think that homosexuality is something that can be "fixed" by dating a woman.
Sexual orientation isn't something that can or should be changed. Everyone deserves to love and be loved for who they are, without feeling pressured to conform to expectations that don't align with their true selves.
The reason why Asmodeus is jealous of Mephisto is because they're the same. They both want to serve and follow their king, and Mephisto thinks that it's Iruma.