You ate the first one and its flesh was sweet (line 5)
Any time you have sweet flesh and the mouth all in one thought, it's pretty easy to tie that to sex. There's also the excitement of the "the first one." It makes the experience new and special, and it sparks a fierce desire in the speaker.
Leaving stains upon the tongue and lust for (line 7)
No hiding it now! Heaney spells it out. Look how he cleverly drops "for picking" on to the next line (line 8), letting us question for just a minute what it is he's lusting after. It's a tricky little move to get us thinking in both directions.
our palms sticky as Bluebeard's. (line 16)
The tale of Bluebeard is a dark one of lust. Check out the "Symbols, Imagery, Wordplay" section for the details. Bluebeard had several wives, and we might assume he's so consumed by lust that it makes him totally homicidally crazy.