I like to think that Carlos wasn’t turning Cecil down before they got together, so much as he just was so absorbed with science that he didn’t realize what was happening.
Carlos: Cecil, you need to tell everyone that the trees will eat people!
Cecil: Neat! I’ll broadcast it. Also, wanna get dinner/coffee/go make out?
Carlos: *looks at science notes confused* No, no, I can’t. I need to figure this out.
Then hours later, he lays down in bed and closes his eyes and replays the day’s events in his head and comes to the horrible realization that he accidentally turned down the guy he liked and just goes “…fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuck”.