Also Known As being a Fuck Buddy.
Entering into Fuck Buddy status without any prior discussion, or, more to the point, assuming that the other person is going into it with exactly the same attitude as yourself, is often directly comparable with shooting yourself in the foot with a rocket launcher.
For the most part, at least one person in a session of platonic making out tends to find out they no longer feel quite so platonic about the object of their lust afterward...