I'm still veering towards "different books are set at different points in time" theory. Clark and Diana dating early in both their careers would be perfectly fine, with hopefully both realising that it's not what they want, but remaining good friends and allies.

See also Samaritan and Winged Victory dating in Astro City, both great heroes in their own right who go on a date because their friends keep suggesting they should, but both quickly realising they're not compatible that way.