I have been wanting to ask this question for a long time and am finally going to do it. From the moment that Lincoln started dating Miss Todd, he knew that her family was a slave holding one. I know they were also fellow Whigs, but it would seem to me that, if he had a lifelong hatred of slavery as is claimed, he would have wanted nothing to do with Mary and her family -- and should have run the other way!
The fact that he did hang on - and come back - would indicate to me that he truly loved her. If not, it would make him a self-serving, political climber in my mind. I would prefer to believe that love overcame slavery obstacles.