"Allow creation..." makes it possible to use the relationship to create related records. It does not set up FileMaker to do so automatically. A person or a script still has to initiate an action that creates the related record. For example, if you had a portal from table A to Table B with "allow creation..." selected in the relationship for Table B, a blank row appears in that portal where you can enter data into fields and FileMaker, once you have entered data into a field, will create a new related record and update match fields specified for the relationship to establish a new Table B record related to the current record from Table A.
Just because you have a relationship line where both ends of the line end in "crows feet", it does not mean that you have a many to many relationship. It just means that there isn't any field option specified to enforce unique values for that field in either match field specified in the relationship. To see a single line "connector" select "unique value" on the validation tab in field options, but you may find that your relationship and design of your database works just fine without this and don't limit a field to unique values without first considering how that will affect the function of your database.