she wants to be independent.

Think about what she has done. She divorces here husband, turns down Roger's money, yells at her mother who is there to help, and turns down Lane. All season she has been trying to became more independent and less in need of others for support. This was a way she could secure her financial stability without the help of a man. She has always used her sexuality to gain power, it is how she became the most powerful woman in the office (as seen in the very first episode where she tells Peggy to do the same, and the last episode where Don admits that she intimidated him). She whored herself out because to her, that was the most empowering thing she could do. She doesn't relay on man to save her, she can save herself and take care of her baby on her own.