A person who has a history of intense but unstable relationships in which they alternate between idealizing the partner and then devaluing the partner, and then frantically try to avoid real or imagined abandonment by others, even if the abandonment is only a friend's brief vacation, describes a person with .

borderline personality disorder

Explanation:

Based on the information being provided it seems that this is describing a person that suffers from borderline personality disorder or BPD for short. This is a mental disorder which is characterized by unstable moods, behavior and relationships. Individual's with this disorder tent to have intense and unstable relationships because they fluctuate and characterize the relationship as either being all good or all bad in different moments. Such as described in the question.

answer; researchers typically cannot control people's sexual behaviors as would be necessary in an experiment;