1) About you being less attracted to her, due to familiarity. What did you expect when you got married? A life-long commitment comes with *surprise* a rather intense amount of familiarity. If familiarity is causing you to be less attracted to her, then YOU (yes YOU) have a responsibility as her husband to find ways to make your relationship fresh, exciting, and new. If familiarity is a problem for you, you will never be successful in a marriage to ANYONE.
2) As for her gaining weight making you less attracted to her. Guess what? ALL of us, in the end -- you included -- are going to end up saggy, wrinkled and liver-spotted. That is the way life works. When you sign on to a life-long commitment -- which marriage is supposed to be -- both you and your partner will not stay 20-something mean, lean fightin' machines forever. Get past the shallowness and learn to love your wife for more than her appearance.
I personally feel sorry for your wife that she feels she has to go online to find sexual and emotional gratification.