To motivate himself to get back into shape, a former college athlete proposes to his off & on girlfriend , only to find out she only said yes to impress her to woo her high school sweetheart.

You have two characters in this logline, the problem is, neither one is likable.
You can have a character do unattractive things, but there must be a reason to want them to succeed.
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Here is an attempt to make the lead character more likable.
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“When he is ridiculed by his class for being fat, a nerdy high school gym teacher; determined to get in shape, asks his on again off again girlfriend to marry him, but when he finds out she is secretly in love with his former high school rival, he vows to lose fifty pounds and prove he is the man.”
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Unfortunately, my attempt has two inciting incidents. Here is a second attempt that tries to narrow down the logline a bit
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“When he discovers his fiance is secretly in love with his high school rival, a nerdy science teacher vows to lose fifty pounds and prove that he is the man.”

Possible title: “Can’t Win for Losing”