Brown Eyed Girls - Warm Hole (yeap, that one).
Gain from the group also deals with sex pretty much every time she releases a solo song.
Gain - Fxxk you
This one isn't openly about sex, but she tells the guy to fuck himself over and over.
Uhm Jun Hwa - Invitation
One of the most iconic songs from the queen of kpop, composed by JYP
She also has Come 2 Me. Never found a translation, but the bridge is in English and it's pretty much self explanatory:
One J-pop singer who deals with sex frequently is
Ai Otsuka - AISU x Time
Spoiler:
If the words I’m about to say become a sign
Then the fun time is about to start
Kiss my wounds, so that they will heal soon
Even when you feel guilty from time to time
I want to touch these petals of love again and again.
Can we see each other again, even though our secrets are so serious? Kiss my nose again and again
You are a "witch"
wow wow, I burrow in your underwear
You are completely at a loss because of me
Your mails drive me insane
Your quiet Love Game is a hypocrisy・・・
Seeing your soft lips
From the edge of my fingertips
To the right of that, to the left of that
To something above that, above that
Our eyes met
Then suddenly above that again, again
I left a kiss on your forehead
A clock that is entirely without hands
Ah Ah, this game of our fingers brings back memories which drive me crazy
Below the centre, below the centre so that it slips, it slips
Show me your tongue if we look in the eyes of each other...
Then suddenly below that, below that you trailed a series of kisses
The moment the clock’s hands overlap
If I gaze at the star within your eyes
It makes me wanna cast a spell on your heart
If the words I’m about to say become a sign
Then the fun time is about to start
If I gaze at the star within your eyes
It makes me wanna cast a spell on your heart
If the words I’m about to say become a sign
Then the fun time is about to start
You can’t say words which are true
You can’t hear words which are true
This flame is burning against a strong wind
and is on the point of dying
You can’t see things which are true
You can’t recognize things which are true
Our hearts need each other
Deeper and deeper...
Overlapping in a way that it thaws
Again, they separate
We embrace each other
One×Time