It sounds like she is suffering low self-esteem, possibly because of a loss of sexual desire, because when women tell their boyfriends they'd be okay with him getting another girlfriend for sex, they usually don't mean it. They usually say it because they feel guilty and really care for the guy.

Is it possible she is suffering a loss of desire due to a physical or psychological reason, rather than losing interest in you? You don't mention her age. Is she old enough that menopause is a likely possibility? Is she on birth control or other medications? Those things can reduce desire. Is something bothering her, that you know of? Depression and stress can definitely cause low libido.

She probably should talk to her gynecologist, who might be able to determine the cause of her lack of desire. S/he may also be able to help increase that desire.

I really wish you and her luck.