Is feeling that relations between the sexes are better than their absence, as a result of watching TV series that contain such relations, considered disbelief, and has sin caused this feeling?

The questioner should have striven against her own self to abandon watching the objectionable series and occupied herself with rectifying her condition with her Lord by adhering to obedience and abandoning disobedience. As for the whispers of evil thoughts, they do not reach the level of disbelief (kufr), because they do not amount to considering the forbidden as permissible; they are merely temptations.