Honesty is best.

Why not tell her that you don't find her as attractive as you used to?

That's not being shallow, you need to have at least some physical attraction.

I like that you offered to start goin' to the gym with her, this way it shows that you're willing to put in the time and effort as well.

And hell, I'm sure it's not like you couldn't benefit from workin' out anyway.

Just don't bring this up when she wants to do the deed, that'll be like a double whammy for her.

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Heil Frasbee