Some people are simply terrible at gauging situations, body language or even giving a shit unless they're told specifically that they have to get active enthusiastic consent. When Antioch College declared that consent had to be actively given the young men were soured and even said "then I won't get any." Scores of redditers recently came out and talked about raping women in that thread from their side. How many of them just assumed they had it when indeed they did not?
http://feministing.com/2012/08/01/the-wednesday-weigh-in-reddit-rape-thread-edition/
This is the kind of rhetoric that ignores the whole point of consent.
The fact that we have what conversation? The fact that more men should actually engage their partner? I don't need to make a woman sign anything, I am a woman.