I'd use it for a variety of purposes, self-gratification being only one of them. Let's also remember on the sex topic- if you are in there getting shagadelic with a hologram, it's not having sex any more than beating off is having sex.
Now that we've covered the sordid, I'd ALSO use it as a bridge simulator to hone my skills as a Starfleet captain in training, an exercise machine (lots of unarmed combat scenarios, like Worf, where I could train to James Bond levels of lethality), a research tool, and a way to visit places in virtual that I want to see and have never been to, or had been to and wanted to revisit. I'd also use it for gaming, D&D style stuff, and fighting the BoB like O'Brien and Bashir.
Taken the to the next level, why wouldn't the homes of civilians BE holodecks? You could then live in whatever environment you wanted and change the scenery as desired. Wake up in the morning in the forest with the birds chripring around you, then change it to an 'ocean swim' for your morning exercise, followed by breakfast on Risa. Clock in to work virtually, and swap scenes again at knockoff time. Realistically speaking, people would hardly ever have to leave their homes for anything.