John had always been a little odd, a little different from the rest of his brothers, even his twin. He'd started to notice it during middle school, when boys started getting crushes on boys and girls, Virgil being no exception, but he didn't feel anything of the sort. So John, being John, tried to figure out why, so he tried to force himself into a relationship with a girl in his class, nothing happened, so he tried a boy in his class, again nothing. So John gave up, figuring it'd eventually hit him like Gordon had done earlier with a fish.
As high school came about John drew more into himself, pushing off advances and gaining curious looks from his brothers, only to stick his head inside astrology textbooks at every opportunity, he started to get the idea in his head that he wanted to work in space, become an astronaut like his father had been, and work up where the stars were calling his name.
He's always been happy looking at stars and studying them, stars didn't judge you for not being in a relationship, they didn't care, their beauty and mystery made feelings rise in him that he hadn't felt in a place other than his family's arms.
He'd told Virgil about this, being twins it was very hard to hide anything from the other, and virgil had gone and done research, looking up the feelings John had told him about and finally coming to a conclusion that he presented to John "asexual aromantic, a person who doesn't feel sexual or romantic attraction". John had been thrilled that there was a name for what he was, that he wasn't broken as he had feared.
The rest of the family had been mainly supportive, Gordon and Alan being too young to get what the concept actually meant and Scott being a little confused until the twins had explained it to him, his father being okay with whatever makes them happy.
John smiled down at the Earth from Thunderbird Five, completely content with where he was in the universe, with the only vaguely romantic love in his life, the stars.