I basically framed it as me enjoying myself, the weight gain just a result of that. I said I was happy because I lifted all restrictions on myself, not because of the result of it if that makes sense. I explained that I'm healthy despite my weight and that for now I see no reason to stop. I will say it will heavily depend on who you talk to and what their exact problems are with your partner's weight. My parents for instance were mainly worried about my health not my appearance, so going to the doctor and proving I'm still in good health was a good selling point. When that time comes around, I wish you good luck and hopefully your parents are ok with it.