tell me why any guy would continue to disrespect his significant other in this manner?
Because he does not care about you as much as you think he does. Or maybe he just wants you to know that he is giving up all these other women for you. Or it could be that he just has a lot of female friends. If this bothers you, then you need to talk to him about it.
Having a lot of female friends is not necessarily a problem. I too have female friends on FB. They are all friends that I have known during my life. We exchange birthday greetings, post funny notes back and forth and that is about it. I spend way more time being with and talking to my wife than I do chatting with all my female friends combined. So the real questions you need to be asking are these. Are you his priority? Is he flirting with them repeatedly? Is he going out to meet up with them without you?
As long as you are number 1 and he is not showing you disrespect by heavily flirting with them or getting together with them, then the number of female friends he has or the fact that he tends to continue to add some should be of little importance. Does he also regularly add male friends? If so, then maybe he is just a friendly guy. In any case, if you are in a relationship, you should be able to sit down and discuss your concerns with him.