To answer this question, it might be helpful to think about what we mean by the word "freak." Officially, the word "freak" refers to an unusual, abnormal or irregular person. It can also be used in reference to abnormal or unusual behavior.
If we apply this definition to the poem, it becomes clear that the title is significant for two reasons. First of all, the speaker presents a romantic relationship which she clearly believes to be abnormal or 'freaky.' In this relationship, the male is completely driven by his own lust and his sexual advances fail to arouse any desire in the female speaker. She describes his advances, for example, as nothing more than "skin's lazy hungers," implying that he is only interested in satisfying his own lust instead of making a real connection with her. So, the speaker thinks that she and her mate are freaks because they have a relationship which is completely loveless and lacks any real intimate connection. It is, therefore, completely abnormal and irregular.
Secondly, towards the end of the poem, the speaker calls herself a "freak." We learn the reason for this in the next few lines:
To save my face, I flaunt, at
Times, a grand, flamboyant lust.
In other words, the speaker is forced to act lustful, like her mate, even though she does not really feel that way. In fact, she feels empty and unloved and, at times, repulsed by her mate's physical appearance. She is truly a freak, then, because she must act against her true impulses and this, she acknowledges, is an abnormal form of behavior.