Even though sex and gender are as different as sex and sexual orientation, (sex being objective and gender being subjective), President Obama has made them synonymous in his Title IX call to force all schools to allow transgender children use the bathrooms of the opposite sex if their "gender identity" matches that sex.
I think it only fitting to apply that same logic to selective service. Having no objective test, men who claim to identify as women should immediately be exempted from selective service.
On a related note, all past prosecuted cases of sexual discrimination should be reheard after the sexual identities of all parties are ascertained. If women were discriminated against, it is important to know their gender identity to make sure discrimination really happened.