Scorpio and Sagittarius make a lovely friendship at the beginning and if they could keep it as such, this would work as a relationship. They're very different. Scorpio is intense, very deeply feeling. Scorpio is the sign of needing a lot of control, a lot of, let's say, a strong, demanding, possessive presence and Sagittarius is anything but possessive. It doesn't want to be possessed and certainly will try to escape the net.
Sagittarius is a fire sign, meaning that it looks for passion in life. The passion is also of the spirit, of the mind. Sagittarius is passionate about being free and independent, about exploration, of looking on life as an adventure. Whereas, what Scorpio is passionate about is keeping and holding what it considers most dear, most essential to its being, to its happiness.
So in the relationship, immediately there is a lot of sexuality because fire and water, although they don't mix totally, they can make the steamy, steamy combustion. They find each other interesting sexually because Sagittarius just adores the intensity and the kind of voluptuousness of a Scorpio. Scorpio in turn loves that quality of enjoyment that Sagittarius can bring.
But very shortly, they're going to run into huge temperamental differences because Scorpio will want to possess, want to keep the Sagittarian in, will feel threatened by the Sagittarian's wide-ranging interests. If Scorpio feels threatened, it becomes angry. It can be cutting, biting, vicious. I don't mean necessarily in a physical way, but certainly Scorpio has a very dark side that can exhibit itself when it feels threatened.
Sagittarius will try to escape and will be very successful at it because it will not be pinned down. So as I said, if they could remain friends that would be lovely. But once they become lovers they kind of are doomed to play this out and then to end the affair.