I have to admit, I'm a little bit obsessed with absolute consent. I totally need to have my wife reassure me at all times that, yes, whatever it is we are doing that I'm enjoying is ay-okay with her, maybe even if it was her who requested that we do it. From sex, to inviting her fat phobic (and, I'm pretty sure, fairly homophobic, although they've never just come right out and said anything about that sort of thing) parents to the wedding, to even just buying myself a new dress (because that's a lot of money that I could otherwise be spending on her, after all) it always makes me anxious and she can relax me just by insisting that she's just as happy as I am about whatever it is. That includes weight gain.
In weight gain fantasy, I only occasionally enjoy the 'spice' of somebody being 'forced' to gain weight, and even then I prefer something like secretly feeding them more than they thought they were eating, or convincing them to eat more and exercise less, maybe by somehow making them think it 'doesn't count', or even just getting somebody to fatten up in a way that is, sort of, against their will but not entirely (like winning a bet against them and having the payment be "put on fifty pounds", which is obviously only a punishment if they at didn't want to get fat when they started gaining the weight, or hypnotising them so that they will stop going to the gym and start eating whatever you offer them and never wonder why they are suddenly gaining weight, or a loving feeder getting their self-conscious girlfriend - who doesn't really believe them when they say they like her bigger body - to agree to let them control the girlfriend's diet and exercise regime until she reaches whatever weight the feeder chooses, only for the feeder to, of course, make her quit the gym and gorge, as she gains and gains, not just in weight but also in self-confidence, and start talking about setting the goal weight above 500kg, to start with, only to keep moving it upward as she grows).
More often, and in real life, I like it when a person is willing and actively consenting to their weight gain. Well, don't we all find it more interesting to see a person (woman in my case, maybe not in yours) gorge herself in public, because she knows that any weight she is gaining is a beautiful thing? Any force-feedings I do with my wife are very much roleplay, although the food is obviously real. Her current weight, is at the point of, shall we say, limited mobility. She can walk (or rather waddle, I should say) but definitely not for long walks on the beach, or anything. Basically, the best way to illustrate it is this: if we go to a buffet, she'll walk in from the car and sit down at the table to catch her breath while I get her food. And unless she desperately needs the bathroom, she won't stand up again till we leave. (Well, then again, I guess we did that already before she was even clinically obese, so that she wouldn't burn calories.)
Now, if my wife asked me tomorrow to help her lose I would totally agree to do so. On the other hand, if she asked me what I thought of her weight, completely without judgement, I would tell her that she has always looked beautiful and sexy, but also that every single gram of that woman is beautiful and sexy, so I'm ecstatic to have so much of her.
If she were to ask me, in all honesty and without expectations or promises, what size I would like to see her at, I would tell her that this is a trick question, because she's asked me before and my answer is always the same: I would love you at any weight, but I would always love to see you get fatter.
And if she pressed on me to give her a new goal to start really going for and planning on obtaining, bearing in mind that we would both only be trying this if we were both happy with the attempt and the results all the way, I would tell her what I have told her in the past:
"If you would be willing and happy to be completely and totally immobile, to literally get too fat to get out of bed without heavy machinery, I would love to help you make that happen, and I would love to help you every day if you did decide to make it happen, and I would love you for making it happen."