Do you think I should tell my roommate and his girlfriend that they can't approach my business partner sexually?
Interesting, is there reason why you don't want your friend to approach your business partner?
It will be immensely compromising for me.
It's understandable to not want to mix your personal relationships with your business ones. Setting boundaries is important at times. If your friend truly respects you, they'll honor your request as long as you're reasonable about it.
I'm just going to demand that they don't do it. That should get the point across.
What you're asking is reasonable but what you're saying is rude and forceful. Demanding things from people is not a good idea and will likely lead them to do what you don't want out of spite. Instead try explaining your reasoning and asking in a straightforward but polite manner.