Headcanon 3
Commander Nebula personally wanted kids of his own, but since he was too busy running things at Star Command and trying to help protect the galaxy, he never found the time to have a love life and start a family, thus why he acts like a father figure towards his rangers, especially when it's necessary and cares about their safety on their mission.
One prime example was when in the movie "The Adventure Begins", when Buzz is eager that he can work alone and doesn't want Mira or anyone as a partner (For valid reasons, he doesn't want to lose another partner like he "lost" Warp) and wants to go on a mission by himself, Commander yells at him, "YOU ARE NOT GOING THERE WITHOUT BACKUP!", in the way a father who cares about his kids would when they want them to be safe.
So yes, that's my headcanon, Commander never had any kids of his own but he continues to be like a father figure to his rangers.