Never Ok. Never Ever.
In a relationship whatever the relation is husband wife or partner or any other relation guys shouldn't slap lady. This is the biggest cowardliness. A real man will never hit a women.
Naturally women are physically weaker than man. People who want to take this as an advantage then his a min minded coward. Because everyone has self respect. Even if you slap your women in a private room you are not only hurting her physically but also hurting her feelings and dishonoring her. So this is a grate attack on her self respect. There are ways to make someone understand about their mistakes. Slapping or physically hurting someone is not the solution. Specially when it is a matter with a female person. No religion allows men to slap or hurt women. In case of punishment for any types of crime is different cause that is only implied by judges. But personally none should slap of hit a woman.