It's totally not a big deal in Seoul. I live there sometimes.
I go to a barbershop every week to get a good head shave (been shaving off all my head for a couple years.
This shop is crowded with mothers bringing their kids, and other assorted people. waiting area in the front, desk, then 2 rooms in the back. the larger one is for haircuts, the other room is for massages. Ladies at the front desk do the massages. Just curtains separate the room from the haircut waiting area and you go right through the waiting area to the massage room. HE is performed at the end of massage. The girls are not particularly quiet about it and sitting in the front, waiting for your haircut, you would hear the squishy noises and the girl encouraging the customer.
The funny part is when both girls are giving massages, and nobody is manning the front desk. It the girls are just doing massage they go out and greet the customers when the bell rings. If they are giving HE they tell the new customer wait please. The moms in the waiting room blush a little, but thats about it. And its normal to pay for your haircut, massage, and tip at the front desk, with the girl asking how you liked the massage. Overall its just done in a matter of fact manner. Usually the girls approach the customer during his haircut and ask him if he wants service after the haircut.