I don't think it's lame or immature to not feel attracted to the person anymore if they gain weight. The weight gain probably brings on a bunch of other nasty things that contribute to the decline in the attraction department. In my view - it's not immaturity, it's human nature.
depends on how big she gets....theres those chics that r thick but curvy ...overall sexy...now if they loose all that and end up being all humpty dumpty fat ...then well.....thats were the benefit of not being married and tied down comes in to effect !!