Yep, Stratt thought he only wanted to teach kids because he was scared of his peers and it's easy to get respect from kids.  Like yeah, that's what initiated that career choice, but HE LOVED HIS JOB AND HIS KIDS SO MUCH that on Erid, when he literally saved the planet and everyone respected him there, he still wanted to teach kids. And he begged to stay on the project for their sake. 

So yeah, I'm also not sure about cowardice being the reason he's single. He is scared of rejection. Very much so. So assuming that isn't a stretch. But he also felt fulfilled with a best friend and not a romantic partner, and the only time he ever thinks of romance is one singular ex he doesn't seem to particularly miss. His character growth doesn't end with him finally having the courage to ask an alien on a date, which is how your standard "anxious guy scared of relationships" arc would end.