I was in a 5 year relationship with a girl where 4.9 of those were more or less stable and all around great times. Then she changed jobs and started interacting with people that made her want to rebel against pretty much everything. Similar to your situation she came to me and proposed that we take a break for a while and see other people. I was pretty against this idea, call me selfish or whatever, but I knew that the break would be permanent. Fast forward to a few months later and we were broken up anyway. Immediately upon breaking up she rebounded with several guys which was pretty painful to see but overtime I got over it and am now in a happy relationship with someone else.
Moral of the story is take the break, you will be thankful you did believe me. If in the end you come back together I think you will be all the better because of it. If however things don't work out and the break turns into a break up then you will have prevented you, or your S.O, from taking drastic measures that would cause the break up anyway(ie. cheating/lying)
I wish you guys the best of luck