You hit the most important part. Communication, openness, boundaries and a willingness to try to work with a person's kinks and desires.
It is a-ok not to want to bring a 3rd person in. It isn't for everybody, and if a vanilla woman finds herself enamored with a wildly experienced or crazy male lover.
The key is to talk talk talk talk. Don't shame, don't go eww, if there are hard lines. Say no (like you did) I am not comfortable with this, but I am comfortable with everything else.
You aren't less of a woman for doing so. You *ARE* less of a woman or to craft Sarah Stroh's Bad Girlfriend, if you lie, do something just because he asked, and aren't clear about your boundaries. He is bad if he guilts you etc.
So my advice if your guy desires you to be with another person and you are like yup nope no way Jose. There are fun little things you can do to give the illusion when he is really really good.
Look up fake cum lube. Tell him you are going for a girls night out (you can make this real) and then later call him and state sweety I hope you don't mind I found my perfect guy that wow just wow. Drink a few more hours and then go back home. Before you go in, inject a bunch into you and smear some on your panties.
Then go in and surprise him ... tell him to give you oral. That might be enough to maintain integrity in the relationship and add the spice you need to make him go "OMFG" who are you sweet baby jesus.
You can whisper see he didn't wear protection either I am sooo sorry.
The best organ is between the ears and the best intimacy is when the two hearts needs are met and maintained with equality, consent, and curiously wonderful imagination.