That's not a joke... I just typed in that site, it's an actual website... wtf...
It's nothing new. Several other sites for this have sprung up in the past few years to basically cater to...
- rich guys who want to fuck really good looking women and having them as long term escorts for events and such, but don't want to marry them (or are just getting something on the side)
- really good looking women who aren't looking for an actual relationship but want the compensation they can get for being with a wealthy suitor
It's... not really prostitution because it's not about the money, but the lifestyle. A mistress will often get compensation in the form of gifts, apartments, cars, or tuition payments and gets to enjoy events that would be beyond her means. Since it's not money directly for sex, it's not illegal. More to the point, everyone knows what is going on so no one gets any real attachment and ether party can move on at any point. There are a few sites that even reverse the paradigm, where it's older women looking for male escorts.
Honestly, I don't have any opinion on this except to say that it's just another example of how the law is willing to look the other way if you are rich.