What we've found is that there are a fair number of guys that cannot get and/or maintain an erection in the same room as their partner. That has lead to separate room play in a number of cases for us.
The Mrs has no problem enforcing condom use and it is the expected norm at parties we attend. Someone trying to slip in unnoticed sans condom would be barred from the parties we attend, and likely others where condom use is the norm.
People are of course free to negotiate condom use so long as both parties are aware and agree, but otherwise your, "Just say No" approach is about all you can do, other than setting expectations before meeting with anyone.