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Because Simon thinks that he has big, ugly toes,
he insists on wearing socks everywhere that he goes at the beach,
in the shower, or around the house, you will find Simon with his feet covered.
The comma between toes and he is completely correct. Because Simon thinks that he has big, ugly toes is a subordinate clause. A subordinate clause cannot stand alone as a complete sentence. You must connect it to a nearby main clause. He insists on wearing socks is the necessary main clause.