Woah, I think this anon is on to something.
Because discovering the hymen healing magic made her go crazy with lust.
Why is Tohsaka Rin so perfect?
No. They were born when Rin and Shirou never even met. They were around 10 years old.
It's just an acknowledgment Rin's a girl's self-insert while Shirou is a guy's self insert.
Not the first time they were both used as main characters, though GO has them being a lot more indirect.