Bella is special. She is the only girl he's ever truly liked, the only one who's felt right for him. Theres never been anyone who's ever felt like her. Other girls can't make him feel what Bella makes him feel. Maybe it's because he can read minds, because of his powers. With every mind screaming at him 24/7, you can imagine he'd be grateful for a break. Bella IS that break. Things are relaxed around her. That's why he's never liked girls. He just hates hearing their selfish, needy, hurtful thoughts.
It's got nothing to do with the things he felt in 1926, the year he went to that all-boys school for poetry and fine arts. It's got nothing to do with the things he thought in 1947, when he went to that gas station where he saw a boy "around his age" working on the engine of an early model of a car. It has positively, absolutely nothing to do with the things he was called when he looked at his friend the "wrong way" in 1962 when he went out for lunch with him.
It has always been because of his painfully impossible-to-stop stream of information. Hearing women "tell" you about their want for you is a major turn off. With Bella, she keeps hers within her control, and tells him what he needs to hear only. It's so much easier to ignore exactly why he's never had a girlfriend in his seventeen years of life, and one hundred years of "life" after that. It's so much easier to tell people that too (not that it's anyone's business but his own). For now, he will just keep this to himself, and pack it away for... Forever.