Why do oral contraceptives lead to a decreased sexual libido for some women?

Why do oral contraceptives lead to a decreased sexual libido for some women?
 
  a. Oral contraceptives raise levels of Sex Hormone Binding Globulin (SHBG), a substance that binds testosterone and makes it inactive
  b. Oral contraceptives raise levels of Sex Hormone Binding Globulin (SHBG), a substance that binds testosterone and makes it more active
  c. Oral contraceptives raise levels of Sex Hormone Binding Globulin (SHBG), a substance that binds estrogen and makes it inactive
  d. Oral contraceptives raise levels of Sex Hormone Binding Globulin (SHBG), a substance that binds estrogen and makes it more active