Buds with Boundaries
The opposite of friends with benefits. While friends with benefits revel in the sexual tension between them, buds with boundaries do their best NOT to have sex with each other. Their objective is to be cordial and friendly with one another, at all times respectful of the other person's romantic relationships.
Jill and Jack once were romantically involved, but have since become buds with boundaries, keeping things pleasantly platonic and sex-free.