in my imagination this "birthday party" was more like an orgy party

First, Seungri paid for the expense of all of his guests (it cost $500,000), also these were escorts (not prostitutes) - they are meant to glam up the party, make sure the guest have a good time (talk or dance with them), he has already done this at his 'Fantastic Festival' party - (I think it was a Christmas party in 2016, nobody made a fuss then, he even talked about the party and the hostess/escorts in Santa dresses on 'Radio Star' episode 507).

This was a party, there were drunk, beautiful people who wanted a great time, if some of the guests (which could include one of these women as well) had sex later on, that's up to them, people can do whatever they want (having sex on your own time, of your own volition is not a crime - quite a few people must have had sex after the party, are they all going to be accused of prostitution).