Rin Tohsaka is straight though. The reason some people say she's bisexual is mainly because of the H-scene in the fate route. But it's clear that it doesn't make Rin bisexual:
First of all, the author Nasu didn't write that scene. As noted in the complete materials, Nasu didn't actually write the script for the fate sex scene.
1.Why didn't Rin tell Shirou about mana transfer earlier? There's absolutely no valid reason. Transferring mana would save Saber's life (and before the whole scene Rin had told Shirou Saber would most likely disappear due to lack of mana and gave him no answer when he asked if she could be saved), so it would be logical to inform him of such possibility as early as possible.
However, if mana transfer involved sex, she had a very good reason not to tell him about it. She had feelings for Shirou, so she definitely didn't want him to have sex with Saber, unless it was absolutely unavoidable, causing either Rin or Shirou to die.
2. The whole scene seems like pointless fanservice. The fact they went for the H to give mana to saber instead of making Rin the master of Saber and let her get lots of mana still baffles me after all this years.
3. It is a life-or-death scene in which Rin is forced to have sex with saber and enjoys it. The question would be be, does it make her bisexual? When 2 girls in an eroge touch each other and enjoy it then they’re bi?
I don’t think so. I strongly believe the result would have been the same with every single heroine in the game. It’s an eroge and I’ve read a few VNs where 2 straight girls enjoy sex together. I’ve also watched even more hentai (not that different from eroge) in which straight girls (usually sisters) that love the same dude end up having sex together and enjoy it very much. In my opinion it was mainly fanservice for an eroge game, not to mention the newer versions of Realta Nua completely remove that scene, making it non-existent.