There are a lot of things people do because they love something, and figure that's how they'd like to make a living. There's a difference, though, between loving someTHING and loving someONE. So that's one point.
Something else is there's a difference between something someone "first does" because he loves someone, and then later does (with someone else) for money.
Most people who "do a certain thing" only with someone they love, don't then do the same thing "for a few close friends". That's another difference.
The big difference is, of course, that the one thing prostitutes do for money is just that one thing. It's a completely different thing from writing, and it's something that comes from a completely different motivation than any other professional does. Writing can come from intellectual, emotional, spiritual, altruistic, artistic, financial, or other motivations. The motivation for prostitution is financial (or, I suppose it could be physical in some cases). Another motivation for prostitution can be intimidation by a third party.
So, basically, (and maybe this is "just me") I think the quotation is stupid. :/