No we do not believe that it is a sin to not accept our doctrines.  
 
"We claim the privilege of worshiping Almighty God according to the dictates of our own conscience, and allow all men the same privilege, let them worship how, where, or what they may." (Articles of Faith 1:11) 
 
Our Church respects everyones right to worship (or not worship) God according to there conscience.

So this almost sounds like you believe there is not absolute truth. ex. If you call the Mormon church the "one true church" why would it not be a sin to not accept that, and go on worshipping through another church?

There is absolute truth. We absolutely believe that the LDS church contains the complete truth that God has revealed to the earth. 
 
However, if someone has not been taught truth then they cannot be held responsible for obeying it. We are accountable for the sins we commit based on the light and knowledge that we have. As a result - people cannot commit sin if they don't know that it is sin. In that sense they live in innocence. 
 
That's why we believe in the afterlife and that all of God's children will have the opportunity to be taught the truth and then make the choice of either accepting it or rejecting it. Just because someone has lived on the earth does not mean that they have had the opportunity to hear truth. But that person who has not had that opportunity while on the earth will have that opportunity in the Spirit World after they die. 
 
This same principle applies to those who have lived any place or time on the earth and have not had the opportunity to be taught the truth. All will have that opportunity after they die. That is why we offer the opportunity to be baptized by proxy for those who have already died. For those who have died they then have the opportunity to be taught the truth in the Spirit World (aka hereafter) and then make the decision and choice of whether to accept the proxy baptism that has been performed in their name. Just because we perform baptisms for the dead mean that that baptism is in effect. We believe that those who have died have to explicitly accept the baptism ordinance for themselves or reject it. 
 
Heavenly Father is fair and just. He has planned for all eventualities and will provide fair opportunities for each and every individual of His sons and daughters. He loves us all.

While we know that the Church is the fullness of the Gospel and the Christ leads this Church through his chosen Prophet our President Thomas S. Monson, at the same time we like Christ respect the agency of all mankind.  
 
To not accept the Restored Gospel of Jesus Christ is not a sin many will be deceived by the cunning works of men. Heavenly Father and Jesus Christ still love these sons and daughters though and have prepared a mansion for them in their Kingdom based upon how well they obeyed the law they had. All will have the opportunity in this life or the next to receive the Restored Gospel and accept or reject it.  
 
Based upon this there character and actions on the Earth they will inherit a Kingdom of Glory based upon the Judgement of Jesus Christ. Without the restored Gospel you cannot receive a fullness of joy but also without it you are not damned as the false religions of the world teach hatefully teach the world.  
We know the truth of Heavenly Father's plan and it does not center upon damnation, but upon the infinite atonement of Jesus Christ and how it brings the salvation of all mankind.