You are mistaken. Explicit looping can be implemented with labels ( : ) and either or both of the branching (b) and testing (t) commands. However, a sed solution does not require any explicit looping, only a single substitution command:
sed 's/\([^,]*,[^,]*\),/\1 /g'
N.B. The sed solution never alters the number of fields in a record. That AWK solution will add a comma and an empty field if there are an odd number of fields (i.e. an even number of commas).