You definitely need to rethink this idea, because it's insulting at the moment. Don't ever use the below phrase.

For people who want to get their significant other in shape

If you want to focus on this niche, use the name in your link, stay fit together. You encourage people in relationships to get healthy together. You do that because they're a team, they're partners. They both decide to take up a challenge to improve their health and overall physical fitness.

For example, you create an app that has a 30 day challenge, where it has a daily workout for him, and a daily workout for her. Mix in couple exercises and stretches too, since they would be a great way to bond. It would help them get in shape, and grow their relationship.