I need to know someone well enough to a) be able to trust what they say about their STI status and discuss safe sew with them and b) feel secure in the fact that they will respect my body boundaries, pronoun preference, and gender identity. I tend to have an easier time getting all those bases covered when I'm in a relationship with the person.
However, I'm polyamorous, so I still date multiple people at once sometimes, and that rocks, too. Best of both! =P