Cancer and Sagittarius have very different energies together and apart. The Cancer, in life, is looking for security and something that he or she can build. Build a nest, build a home. In astrology, Cancer is considered the sign of the mother and the home. Not meaning that everyone becomes a mother, because the male Cancer certainly wants to create a home. But there's a mothering instinct in all Cancerians, a protectiveness, and a willingness to reach out and draw in.
Sagittarius's energies are completely different. They want to adventure. They want to be free, to be free spirits, to explore, to live life as if it were a rock climbing, let's say, adventure. As if they were para-sailing. Not that they necessarily do those things, but that's they feeling, flying outward, being open and not being tied down.
So at first, the sex between them is very interesting, because Sagittarius loves to play at sex, and certainly Cancerians are wonderful at giving sex. Even there, however, pretty soon one tries to escape. One tries to hold one in. And so the Cancer and the Sagittarius ultimately they part ways, because they are looking for different things, and they cannot supply each other with what they need.