The Nine things an Arahant cannot do:

1. Store up possessions
2. Intentionally kill any form of life
3. Steal
4. Perform sexual intercourse
5. Tell a deliberate lie
6. Act improperly out of desire
7. Act improperly out of ill-will
8. Act improperly out of delusion
9. Act improperly out of fear

(from Anguttara Nikaya 9.7)

Not because he/she is unable, but because there is no longer the interest. The Arahant acts in accordance to the four Brahma Viharas (Anguttara Nikaya 9.7). Not doing sexual intercourse applies to non-returners too (Anguttara Nikaya 6.63).

ReferencesEdit

• The Complete Book of Buddha's Lists -- Explained. David N. Snyder, Ph.D., 2006.
• http://www.thedhamma.com/