Blackberry-Picking Theme of Lust

The boy in "Blackberry-Picking" is just about that age when sex (or at least ideas about sex) is starting to become part of life. The desire to gather and eat the berries represents the desire to, well, you know, do something like that but with women in place of berries. Ever heard of the forbidden fruit?

1. What about the timing of this poem makes lust such a factor? Does age have anything to do with it? The season?
2. Why bring creepy old Bluebeard into this idea of lust?
3. Why did Heaney use blackberries to talk about lust? Why didn't he just talk about a youthful romance? What options does the metaphor give for the poem?

The blackberries in the poem could have been replaced by any fruit and had the same effect.