Assume this is your gf. You love her. She loves you back. She confesses that she's been having sex with another guy for fun and she says she has no plans on stopping because his dick is much bigger than yours. She's willing to continue having sex with you as often as possible but not quite as often as most couples.
I would break up with her.
I would let her cuckold me as long as I get to receive a hj or bj after watching her have sex with the other guy.
I would demand threesomes but risk never having sex with her again for being an inconvenience/annoyance.
I have a BBC.