Huh, I thought that (and the creampies) were both from committing to her. In the walkthrough there were definitely places to gain lust points for Devon, so I wonder if was the rumours from other girls, or if it was the one big decision at the subway.
And yes, it's absurdly easy to know that the canon route for this game is to fuck as much as possible, swing and share, and let Bob in on everything. The game is much shorter otherwise (and I think there is just one extra scene, with Debbie, if you're not friends with Bob and go work at the library).