(24M) (23F) My girlfriend no longer believes in monogamy

My girlfriend recently received an injury at her workplace, she's been given compensation and a few months leave to recover.

So far, she's spent her days binge watching TikTok, and has begun to follow some creators with 'unorthodox views'.

She has recently come to the conclusion that "Monogamy is the construct of the patriarchy, designed to limit women's sexual freedom".

My girlfriend has also thrown out ideas that "Marriage as an institution is a form of slavery for women" (bear in mind that before this she kept pestering me about getting engaged).

I was quite confused by all this, so I let her know "You can have an open relationship, it just won't be with me", to which she began laughing and claiming that my masculinity was threatened at the prospect of her being pleasured by someone else.

I asked my mother about all this, and she suggested I reason with my girlfriend, but my gf literally has an answer for everything, she's become some kind of expert in this strange philosophy.

I don't want my relationship to end, I just want my girlfriend (in her normal mind) back.