Lots of people are married and poly (I am one). Was she poly too, and is she giving up other relationships, or was she always monogamous to you? Does she know and/or like your other loves?
I agree with the others who said that giving up something you don't really want to give up is not a great idea and may well lead to resentment in future, or even the danger of cheating if you are making promises you can't keep to.
If you didn't currently have any other relationships, it might make more sense to me - some people do choose to close up at various points in their lives (and may or may not open again later), and as long as that was something you both agreed to then fair enough - but there are other people's hearts involved here too.