However, I don't think the idea itself is horrible, maybe just the way you're presenting it. Look at your app name "get fit together". Doesn't sound like a fit-babysitting service, does it?
It does seem a bit shallow to say "for people who want to get their significant other into shape", but the idea of "let's get in shape together" isn't a bad one, and it doesn't even need to be for significant others, it can also be just for friends. Make little competitions, allow people to challenge each other, etc. etc.
Think of how many people run together, do bootcamps together, etc. etc. how about letting them track or challenge (or whatever your app does) together, but it doesn't have to be like a tattle tale.
$5 bucks a month? Probably not. but somebody might.