If your parents didn't like your bf/gf...

This is what I'd do in this type of situation:

Ask my parents to explain what's wrong with my girlfriend. If it's reasonable, I'll take it into consideration. However, if it's something ridiculous like her not being a Christian or being a non-Caucasian race, then too fucking bad, she's still going to be my girlfriend anyway.

Besides, if the latter of the two situations were to occur, it's not like my parents are never going to speak to me again.

Also, my dad's mom didn't like my mom when my parents got married and over the years my grandma started to like her. So it's not like the parents can't eventually like the girlfriend/boyfriend.

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