It is, in my experience, a predominantly male (and predominantly young male) trait - the tendency to assume that once a certain level of intimacy is reached, that you can get back to it whenever you like. You see it inherent in the "base" metaphor - once you get to second base, you either stay at second or progress on to third - there are very few circumstances in which you are safe at second and would choose to run back to first.

But of course, that's the male sex drive (and, arguably, the role of masculinity as taught by society) at work - it tends to be acquisitive, focused on getting, keeping, and escalating. The female sex drive (or, arguably, the role of femininity as taught by society) is a lot more context-sensitive and focused on individual moments and moods.

But regardless of gender or gender roles, I think we can all agree that consenting to something on Saturday doesn't mean you consent to it on Sunday, and assuming makes an ass of pretty much just you.