This doesn’t need to be a deal breaker. Everybody has their own views on what sex means and that’s okay.

Be honest with yourself and ask yourself if you’d like to be a relationship with somebody that has those views on sex - to the point where they are willing and have a desire for an open relationship.

She should be honest with herself and ask herself if despite her feelings/views on the subject of she will be fulfilled with only a single sexual partner forever.

Have an honest conversation with each other on this subject. She may be the one or she may not - it will just come down to what each of you want and need out of this aspect of a relationship.