Here is the deal.
Rebound sex is fine as long you are in a place mentally where you are NOT bitter towards women. If this is the case, every woman you meet hence forth look like different versions of your EX. Then you treat women Poorly! This is counterproductive and will lead to more BAD relationships.
It can be very helpful emotionally when you are honest about where you are with her and you two enjoy each others company as lovers. This can help boost your self-esteem. But, don’t let it become a drug or a crutch taking over every spare second. You need to allocated most of your time to improving YOU.