It is perfectly normal to fantasize about someone else while having sex. The only wrong thing is if you go out and do it. Do not let your partner know how you are thinking as it will make him feel less of a man and could ruin it. If you are so concerned about his weight maybe suggest he start a gym for health reasons. Unfortunately I gained 45lbs because of an accident and my husband told me he was thinking of others and I know how it made me feel. Some things are better left unsaid!!
