The massage is just the first step of the chain of events. You can still refuse Cassie later in the lingerie shop, but it will prevent you from going on the love path.
I always thought that a player who wouldn't enjoy that kind of content would simply load a previous save and go another path. Maybe the fact that there are several different paths available isn't evident.
One thing is for sure, I should find a way to warn the player so that he can make an enlightened choice. It has been on my to-do list for a while now, and I need to work on that.