Slapping someone is considered battery or possibly assault. Slapping someone’s bum is a sexual thing. So I don’t see an argument for you really. Unless you want to rename it sexual battery which just sounds like something you put in sex toys
"sexual assualt" implies a rape, a woman on a train was in appropriately touched, on leg and ass by a guy 2 days ago, however, the news reported she had been repeatedly sexually assaulted. Someone tells you that they were sexually assault, you going to think someone touched their leg or worse?
No, I would not automatically assume that someone who was sexually assaulted was raped. The definition of sexual assault is just 'unwanted sexual contact'. This can range from sexual touching, such as smacking someones ass, right up to rape.
A person wouldn't get 5-10 years for slapping an ass. Just like murder, there are different degrees of sexual assault. For rape you can get up to life imprisonment, attempted rape or intent to rape is up to 14 years and other sexual assaults can get up to 10 years depending on the level of harm was caused. Slapping someone on the ass probably wouldn't even get you arrested.
However, it's still an act that is sexual in nature and is performed without consent, so it is a sexual assault.
In Spain, a rape occurred during San Fermín in 2016. The case was already mediatic for happening at that moment, but the judge's decision was that it was sexual assault and not rape, so a lot of women protested. And just yesterday, they were allowed to pay a fine and leave prison.
Rape is rape, and this wasn't sexual assault
However, it's still an act that is sexual in nature and is performed without consent, so it is a sexual assault.
Rape is rape, and this wasn't sexual assault