Set your SEX field to a calculation field with the result type 'Text' and the calculation as follows:
SPAY = "Spay" ; "Female" ;
NEUTER = "Neuter" ; "Male"
But, if you still want to be able to set the Sex field without selecting either Spay or Neuter you won't be able to as it's a calculation field. If this is the case then set the SEX field as Text with an Auto-Entered Calculated Value with the same calculation as above, but make sure the 'Do not replace existing value of field (if any)' checkbox is UNTICKED.
Also, could you not have one field where the user could choose between Spay or Neuter? In your setup they could both be selected!
Thank you Phil..............I new it would be simple, I was just missing a step!