Mono / poly relationships are common enough where the mono person usually chooses to be mono.
I have a poly friend who married a woman who can only accept monogamy. He said he will stay with her but he kind of regrets marrying her.
I would suggest asking her to read a book on polyamory. Ask her to make an informed decision about what you are feeling. But in the end, it may still come to a choice and only you can really answer if you think you can be monogamous.