If your using RC1 or 2:
Change your custom user profile field to: "Yes, including a first blank option" ALSO make sure it's editable by the user.
Then in your postbit (or Postbit legacy) put the code:
PHP Code:
Sex: <if condition="$post[field99]">$post[field99]</if>
Probebly above 'Rank' would be best.
Change all the "field99" to the number of the profile field. For example if it's 'Biography' it would be profile "field1". And that should work for you.