Why am I so attached to my male friends yet dismissive of my female friends?

Greetings. You mentioned that you have a diagnosis of Borderline Personality Disorder. Some of the defining features of BPD are unstable relationships, changes in self-image, and a fear of abandonment/rejection. These issues, in turn, are related to an insecure attachment style, most often Fearful-Avoidant. You seek closeness, but you foresee negative consequences, and thus find yourself in a loop of engaging and withdrawing. In therapy, it would be important to explore your historical family dynamics and any attachment trauma that you might have experienced, and to promote your sense of self-expression and self-efficacy.

A common question that comes up in general is: "Can men and women be friends"? My answer to this question would be that friendship between men and women who are attracted to the opposite sex is intrinsically intertwined with that attraction. Therefore, there is always the potential for an intimate relationship to form between them. Consequently, a man and a woman in a committed relationship will naturally be protective of each other. They might not want their partner to spend too much time alone with someone with whom attraction is a natural possibility because it might affect their own relationship with that person.

Finally, we must look at the definition of the word intimacy in the dictionary. You will notice that it is defined both as a sexual act but also as a "close familiarity or friendship, closeness". Therefore the term casual, which is defined as "relaxed and unconcerned" is not well suited for intimacy. 

My wishes for you and to anyone reading these paragraphs are to find a sense of fulfillment in everything that you do. I hope this answer was helpful. Please feel free to reach out for further input. Regards. Vadim.