This issue obviously affects the females who end up competing against biological males. The integrity of the boys races is also affected, if less obviously and dramatically than the girls. This affects the males running in boys races whose effort and times are compared to one standard, while males running against girls gain accolades and status based on an entirely different standard.
A person taking this position could also be transphobic, but I contend that this argument in itself is not transphobic. As some thoughtful posters have acknowledged, a transgender girl should have the right to identify as a girl -- which is her gender -- and be treated justly in her life.
Social identity is related to gender, which is fluid; athletic advantage is connected to sex hormones, which do not change when one chooses to identify the other gender. When it comes to competition, one should be able to legitimately argue without being accused of bigotry that that transgender girl (biologically male in terms of sex hormones) should not compete against biological females