I don't know what he was expecting would happen. It's not like the Enterprise can orbit these planets forever and they can't come with them. Short-term relationship is all he's gonna have.
She'd become part of the crew, so she and James could carry on their relationship. Picard's Data to Kirk's Rayna, except that there's a love interest as well. Rayna would augment Spock as a scientist/researcher/etc. And then of course, you'd have to have an episode where Rayna sacrifices herself to save others and... that would be that. Kirk is single yet again.