Rape is a crime.
The only 'surprise' should be the punishment by law the rapist receives for this crime.
Some examples below:
I shoved my cock in her mouth as a treat. She gagged and tried to back away.
She thought I was giving it her in her pussy, but I went up her arse and pretended I didn't know the difference.
When performed on your girlfriend, it's surprise sex.
When performed on an even hotter chick, it's rape.
Rape is such a harsh word... everyone loves a surprise!
"I'd been following her for weeks, i knew it was here birthday soon and planned to surprise her"