"Well, that doesn't necessarily mean anything when it comes to being with someone. Anyone can on their first time. But I'm glad you didn't. I'm too young to be an uncle, you know."

Not that Keith was too young to be a father. Not that it wasn't something they should risk. Yes... it was because Shiro was too young to be "an uncle". Yes, he was jerking Keith's leg, but it was also a reason Keith wouldn't soon forget if he wanted to avoid Shiro's jokes.