These two are very different from each other. At the beginning, of course these differences will put them into an exciting affair. Pisces is a water sign. In astrology that means that Pisces is deeply feeling and intensely romantic, and loves the whole fantasy of love and sex. Pisceans make fabulous lovers because they can enter into anything that their partner can imagine in a way that can fulfill the fantasy of the sex partner.
Sagittarius of course is gripped by this because Sagittarius loves passion, especially Sagittarius loves an energetic approach. So, sexually they are both passionate and energetic. The difficulty is that Sagittarius doesn't want to be tied down and Pisces does. Pisces wants someone strong in his or her life that will be a partner and a lover, and someone to take care of him or her.
Many times, it isn't just financially, it's emotionally. The Piscean needs to have that communion, a feeling of being in a long-lasting and deeply feeling communion. This is exactly what Sagittarius may flee from because Sagittarius doesn't like having demands.
So, in very short order, the Sagittarian nature, which is to escape, and the Piscean nature, which is to enter more deeply into a relationship find that they are completely at odds. They might work well as business partners because Pisces can set up the creativity, can come up with the ideas and the imagination and the fertile brain of a Piscean. Sagittarius, of course is filled with ideas, so together they can create beautifully.
As lovers, sex is very innovative. As long-term, loving partners they have very little in common.