I agree.
People should look good and be healthy for themselves first and their partner second. I think it's ok in a relationship to encourage and motivate your partner to look and be their best if it comes from a place of love and wanting the best for them - not if it's from a place of what's best for you.
It's not ok, though, to nag someone about specific flaws thinking they'll change.
There's a big difference between the "lets go to the gym and get hot together" sentiment and a "your ____ looks like shit. You need to go to the gym and fix it" one.
yeah, me and the other half critique each others work outs for lagging body parts...and are semi brutal to each other about food stuffs but its mutual and not mean on either end
and when the other makes progress on something that hasn't been so great we're both quik to compliment each other.