They will soon realize that the new person isn’t right for them and they were just being intimate to quench their thirst for intimacy.
And once they do realize it, they will break up with them.
That is why most people (especially girls) will become intimate very soon with their rebound relationship because they are trying to get to that level of intimacy that they had with you.
But usually, the faster the rebound relationship progresses, the faster it ends.
Instead if you do something opposite, and act indifferent to their new relationship and just concentrate on your own life (career, hobbies etc.), it will get them thinking.