I told mine straight out, relationships are made up many things. Attraction is one of those things. If I ever become unattractive to you whether it's appearance or demeanor, let me know and give me a reasonable amount of time to change. If i don't change, I will totally understand but that street works both ways. I can love you all I want, but if i'm not attracted to you, that will be a strain in the relationship and will eventually tear us apart. Let's make an effort to always work out and try to eat healthy. Let's have balanced lives so our relationship can be balanced, etc.
My girl understood and accepted this. Not all girls will. Good luck to you buddy.