I tend to use the pre-relationship tag on AO3, irregardless of gender, if I know the fic I am writing is MEANT to lead to more. (I don't always know. Sometimes a relationship evolves like in the real world, completely unplotted.) Then again, if the 'relationship' is actually going to be just about physicality, UST might be a better choice. The perception is that 'relationship' admits emotional entanglement, whereas UST doesn't have to.

pre-slash has SOME proponents in the f/f category. There are those of use who doggedly tag things as "male slash" and "femme slash" to keep the male from being de facto and default?