Playing SL is not for everyone. But it has several advantages. You can't catch a social disease. You can go on line anytime. You don't have to travel or rent a hotel room. It's much cheaper than real life mongering. And it's unlikely your wife would divorce you for virtual cheating. That is, assuming it stays virtual and doesn't become real.
Still, there is nothing that can replace real life sex. You can't touch the person. You can't feel the heat of their body. You can't smell the smells of sex........
well at least not for now. Maybe in 50-100 years we will have sexbots.