If a guy says about himself, "I know not to cross the line", in 98% of all cases he is aiming for exactly crossing that line. But because there are always those stupid chick, that believe, that their good acquaintance, they know oooooh so well, belongs to that 2% honest ones, let me tell you this: They don't. Make that 100%.
And before someone accuses me of sexism, this wisdom also works the other way around, with genders switched or the same sexes. It's something in the human instinct...What was it again...Ah yeah, right. To wanna fuck, whom you fancy.