Marriage and family therapist here.

Red flag? I'm not sure about that. I'm hearing that there may be a breakdown of communication.

Here's a question; what message did he receive when you asked him to sexually introduce someone else between you two? He was reluctant. Okay. Is he feeling good about the situation? How openly expressive is he, generally? I wonder whether he may have buried his own sentiments, thus, not genuinely expressjng his emotions in an attempt to make you happy?

Would you be satisfied if he came and said that he was utterly not okay with opening the relationship?

I can't say what's on his mind, because i haven't heard it from him, but I get the feeling like he doesn't feel very good about the idea. Is he feeling like he is satisfying you, or is he feeling like you're looking for other attention outside the marriage?

Has he set proper boundaries and properly set boundaries with what he is okay with? I have a feeling that he may be far more hurt by the request than you may have anticipated.

there are quite a few potential points at play here!