Quick HF question(SPOILERS!!!)

[TheOverlord]

I couldn't wait any longer and played through HF using AGTH and understood most of it but i was never able to figure out why

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Rider had dream sex with Shirou at that time. I mean she 100% devoted to Sakura even after she used up all her command spells(Just look at her first command). She new how much Sakura loved him so s****ing him even in the weird dreamworld seemed totally OOC.

With the similarities between

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Sakura and Rider's pasts i could've understood it happening after the true end(I know they do it in HF) espically after what Rider says after Shiro instucts her to give priority to Rin and Sakuras treatment.

Basically any explanation of why it happened when it did is appreciated