Dairy mistress

When dating a vegan, you're likely to not eat animal products when eating dinner with them. However, you likely eat animal products when at dinner with a friend(s) who is not vegan, and this friend(s) is/are your dairy mistress(es).

If your partner is vegetarian, the friend(s) would be a meat mistress(es).

Bob: Dude, I totally couldn't eat this grilled cheese if Susan was here; she became a vegan 6 months before we started dating. You're so my dairy mistress.