Whether you're talking about losing weight, or even alcoholic behavior, I can respect an "I can't be with you, if...", but especially in a live-in situation, an "I'll marry you if..." to me means "you're good enough to sleep with, but not good enough to marry."
I can even understand a man or a woman having to decide that the relationship needs to end, because conditions are being put on marrying that constantly postpone the wedding.
I postponed a lot of my life trying to lose weight first. I would so often say to myself, "I'll date (and do thousands of other things) when I lose all of the weight." I knew I could find nice guys who liked me the way I was, but I didn't want to face the possibility of dating men who preferred me fat, or would might want to marry me before I lost the weight. I also didn't do a lot of things because "I'd look stupid doing them fat."
I spent 30 years trying to lose weight so my life could start, and I finally realized that if I couldn't get all of the weight off, I could spend another 40 years waiting (if I wasn't lucky and happened to die before that happened).
So, I decided to start doing everthing I could that I had placed on the list of things I would do "when I lost all the weight." And that included meeting and marrying my husband.
Could I have had a "better" life if I had been able to lose the weight and keep it off at 16? Probably. Although I also could have had a better life if I had decided at age 10 (or even 5) not to let my weight prevent me from doing anything I wanted to do.
Although, I am finding it easier to lose weight (it's still crazy difficult) while not letting the weight stop me from doing anything the weight itself doesn't stop me from doing (I can't run up a flight of stairs, but I can wear a swim suit and get into the water, even though I look ridiculous).
When you've put 99% of your life on hold, it's hard to give up your few pleasures (eating, for example), because then you've got "nothing" left. It's hard to live on nothing, and no one should have to.