Originally Posted by ViableAlternative
You might add, "If you DON'T respect this boundary, I will be less likely to consider group sex with you, and if pushed, may end our relationship entirely."
Or whatever else you might want/need to say.
You've mentioned that this guy has been nicer and more giving to you than any previous partners you've had, but I really feel like throwing the old 'other fish in the sea' cliche at you.... You really *deserve* to have what you need from a partner. Respect isn't so much to ask, and should be a cornerstone in any relationship. Don't let this guy - or anyone else - use you or fail to respect you.
I am going to use this advice (both on how to discuss this and the respect part).