Any advice where or how to get this so called "Lust art" ?

[Ray]

There are these dance rank and waitres rank but havent being able to find this Lust art rank ? Any idea where or who to talk ?

[Ydyp]

I think there are various methods of gaining this, I know that I got xp for this by interacting with the boy that flips your skirt, same as when going and get yourself captured by the gang of thugs as well as working as a prostitute (once unlocked). So in short you should get xp in this whenever you give sexual service to someone (groping excluded).

[Kuckoo]

You'll gain Lust Art exp by initiating sex acts with other people, when it's done to you forcefully it doesn't count.