Is it normal to get a 'platonic crush' when you are in a relationship?

I'm having a really hard time. There's a guy I find really attractive at my gym, more so than his physique, the fact he knows exactly what he's doing is pretty attractive. I love my boyfriend, but if I'm being honest, he's been gaining some weight and I feel terrible saying but I feel like I'm losing the sexual spark and I feel SO bad finding myself attractive to this person. Is this normal?
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Is it normal to get a 'platonic crush' when you are in a relationship?
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