How do you do that? And apparently I hear Orcs have a unique sex scene via Cock kick? How do you do these things?
To expand on Fapokso said, she needs to fit all the parameters to initiate vaginal sex, then attack with any type of kick, and the enemy needs to survive. Even then it's not 100% guaranteed. The only enemies who can counter are thugs, orcs, lizards, yetis, and some of the guards. After she gets countered 10 times, you'll get an attack called "Light Kick" that can be used even if she's armed and will do very little damage.
The easiest way I've been able to do this is fight a multi-wave fight on the 3rd or 4th level. By the end of the first wave, she'll probably be wet and ready. I then use cock-kick on an orc or yeti. They're usually strong enough to survive the hit, and very likely to counter. I do it this way because the enemies usually get the first move, and if she's ready for vag sex, they're pretty likely to get her into a sex pose. The guaranteed first move at the start of a wave is a way to get around that. Of course, if you have a high AGI build, you'll probably go first, so you don't need all this trickery.