She's just your girlfriend....
Tell her we're all walking, talking, self-important bacteria on a giant rock floating through space that could be devoured by a supernova at any moment. What difference does it make if I stick my dick in another girl right in front of you?
If that doesn't work, tell her it's your birthright as a man to have sex with multiple girls at one time, and if she's not down, then she needs to find a guy who doesn't care about those things.
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