Saiki knows what love is; he's not ignorant. His parents love each other and he's glad about that. He knows some kids have parents who don't love each other so he's lucky that his parents do. His parents love him, even his brother loves him, he knows. However, he could never grasp romance.
Ever since he could think, walk, and talk, he was exposed to romance. Television, books, and fairytales were laden with romance. It was supposedly supposed to be a form of love, but Saiki never seemed to feel like that fit. He loved people, or at least his parents, so he knew how that felt. But he had never felt romance, or if he had, it most certainly didn't feel like love.
Sure, someone could comment that Saiki was being closeminded and not accepting that he doesn't know everything, but he does know everything; everything important that is. So, for him to not know or have experience something as seeming vital as romance just seems like hit to his ego more than anything.
Saiki never developed an outright hatred for romance, more of a growing annoyance. He'd never had friends until high school, and even then, those he grew close to he only felt platonic affections for. He saw his friends' friendships developing into romance before his eyes. But no matter how many times he combed through his friends' minds for answers, he couldn't understand romance.
But, once he gave up his powers, he grew to just leave it be. Sure, he might not understand it, but now that his powers were gone, he wouldn't understand everything anymore. He'd thought about asking his friends about romance but decided to leave it be. They had never ostracized him for his seeming lack of ability to feel romance, never ever seeming to comment on it. Even friends who were originally after him because of romance had let it be and just developed a friendship with him.
So, even though Saiki could never grasp romance, no matter how hard he tried, he decided to let it be. After all, he was already weird, what's one more to add to the list?