I feel sources in English talking about the roots of yuri pay too little attention to BL's role, despite it having had a clearer influence than S. Maria-sama ga miteru is an especially important case—its inspiration was the idea of doing soft shounen-ai but with girls. Oyuki Konno said in an interview in Eureka 2014 Dec: Yuri Bunka no Genzai that she didn't know about S until after she had created MariMite and people were asking her about whether Hanamonogatari had been an influence.
In the case of Shiroi Heya no Futari, Ryoko Yamagishi wanted to do a shounen-ai story, but made it about girls instead, suspecting that would be more palatable to shoujo manga readers. Around the same time, Moto Hagio worked on November Gymnasium, the one-shot that would be basis for The Heart of Thomas, and had an early draft where it was about a girls school, so it sounds like Yamagishi wasn't the only mangaka to have that worry.