
So when did they start dating??

The "hemorrhoid" incident was the turning point in my view. Fuji wanted a relationship with Taichi and had already commited himself at that point. When Taichi made the declaration that he wouldn't give him up, that was the moment he committed to Fuji. From there on, they were dating/in a mutually committed relationship.
http://www.mangago.zone/read-manga/hana_to_junketsu/nbt/54040a7ec526f1931/Ch5/55/
"Him Are you a girl?"
Me:" he might become one if you top him."