Summer Heat by Lawrence Kinden is about a girl who puts her rear on the line to spare her girlfriend, but isn't strictly about them being spanked *because* they're lesbians.
Did the Library give you a reason for its rejection, kerr? I can understand if, in the current climate, they thought that even for the site and kind of stories they feature that would be a bit heavy. That said, I do think Kinden's approach could work, where the queerness is front and center and not *unrelated* to the spanking, but not the reason for it in a way that pulls the story into a similar place as your rejected story.
Say, the usual "teens who missed curfew because they were making out," or any number of other classic tropes which result in teens getting spanked; perhaps even with it being a surprise that the parents are okay with the relationship, but are still spanking their daughters for the aforementioned rulebreaking. | 
