If you are going jump aboard the good ship "Ex Girlfriend for a Reason", then be really careful that the easy, free sex doesn't cloud your perception of where this is likely to lead. A few previous posters have touched on it and it's likely to end in tears. Yours, hers, or maybe both.
If both of you are absolutely clear that it's just for sexy fun then get stuck in. If either of you think it may lead somewhere else then avoid. Experience of many, many people indicates that this is unlikely to go in a positive direction.