Ambrose2007: A more interesting question - one perhaps deserving of its own thread - is: what you do if you find slimness/good conditioning necessary in a partner to feel sexually attracted to them, and your slim partner begins gaining large amounts of weight in the course of your relationship - what are you reasonably entitled to do? Should you end the relationship or speak to the lady/man about your concerns? How could you possibly avoid sounding as though you were presenting an ultimatum in this case (I can't be sexually attracted to you unless you slim down)?
Is the above scenario fundamentally different from the one offered by the OP?
Hmm do you know what I would do if my other half was getting a little 'portly' ..
No?... oh.. okay.. then let me tell you..
I wouldn't EVER
say anything about it.. I may talk about how 'I' want to lose some weight.. and that I want to begin to eat more healthily etc
I would also come up with more things to do out and about together.. walking biking swimming etc..
Enuff until I could say.. 'Wooo baaaybeee look at yoooo.. you are losing weight!.. You are looking even MORE scrummy than before!

It's all that healthy eating/biking' etc.. we are doing.
Anyhoo.. I'd do something like that.. I wouldn't ever say it.