Can't say for sure because I'm not an obeast, but I think it's because at least for some women, once they get married, they feel that they don't have to "attract a mate" any longer and let themselves go. I know for me, I do everything I can to maintain my figure for myself and my husband. Even though I don't have to "attract a mate" anymore, I still want to stay attractive for him (and I know it gives him an ego boost whenever we're out in public and he sees other men looking at me.)