Rin isn't bisexual. Her actions in the Fate H-Scene was a result of her being a normal, healthy, seventeen year old girl with all the natural sexual urges that brings, but she was also severely closeted in her emotional and social growth because of her hidden life as a mage. (when I saw closeted, I refer to the definition of living in a state of cautious privacy, some people only think it means hidden homosexual feelings) This is most apparent in her interactions with shirou during UBW. Despite her status as a school idol, Rin doesn't actually seem very socialized, the way she constantly teases Shirou to see him react or blush is eerily reminiscent of a 10 year old girl tying the shoe laces of a guy she has a crush on together or stealing his lunch. It would be the equivalent of Shirou sitting behind her and pulling on one her twin tails in class. So Rin is EXTREMELY sexually frustrated going into the events of FSN, she obviously wasn't sexually active and had zero prospects on the horizon. She probably wasn't even masturbating regularly, though she knew the principles well enough in fate. Because of this she was moving into a psuedo/situational sexual state where pretty much anything was starting to look good. She was open minded and had nothing against the idea, so it was starting to get the point where any kind of tail seemed like a good idea even if it was a desperate bet with ayako or being star struck by a rather captivating blond knight, that was just a symptom of someone whose normal teenage urges had been clamped down hard by a would-be stone cold magus and it was majorly affecting her.