That saying: "Ein bißchen bi schadet nie" startet
as a joke just like "Wer nicht bi ist, verschenkt 50% seiner Möglichkeiten" (being not bi just cuts down on 50% of possibilities).
Why do you think that's a german phenomenum? Kind of strange thought here... like people saying the sado-masochism scene started in germany (though SS-uniforms are sexy

).
That statistics you mention here aren't representative. For starters they just asked citizen of great cities like Hamburg and Berlin. Out of those populations they choosed members of certain scenes. And they asked just of few hundred to thousand people which isn't very representative in a city of over a million.
Asking people of the remixing community reveals equally useless mark-off results.
BTW.: counting people who aren't sure if they might engage a member of their own sex or not as bisexual is a bit questionable and borders on manipulation.
Technically there is only one scientific point (beside behavior) that leads to genetic preposition - that's the "ur"-gender being only one gender back in the dark days of early vertebrates. This explains very good why there is gay behavior also found in the ranks of other primates. Research regarding percentages speaks for itself there.
Now lets have a look at behavior. Bonobos are very good example where sex in general and bisexual behavior acts as a violence preventing strategy. Same goes with penguins when "appropiate" partners aren't available - social contacts are overriding natural preposition.
Looks like being gay is "learned" in social context. Unfortunately studies about "accidentally" gays through abuse or other traumatic events are influenced by interest groups (and therefor aren't representative).
There is no paradise of people being all straight or all gay.
Had a choir in my heart and had to kill it to survive.