Now tell me what part of not getting the service that you have paid for you think is acceptable?
Not getting the service you paid for is not acceptable. There seems to be some question as to why you did not the service you paid for.
You say you checked in on time. It appears that the flight was not overbooked. So we have an immediate discrepancy here.
When you buy a ticket for any form of transportation, it is a contract between two parties. As the carrier has obligations towards the passenger, so does the passenger have obligations towards the carrier and other passengers.
I'm not saying that this is the case with you, but I have been involved in resolving cases where a passenger has presented himself late, or drunk, or clearly unfit to fly, or incorrectly documented ......... many situations can arise. Then the passenger blames everyone except himself.
I'm not having a go at you, please don't take it that way, but we still don't know the reason why you were denied boarding.