You'll meet Morenn the dryad in the druids' grove in the Swamp. To sleep with her, you'll first need to talk to her about the quest A Lost Lamb. Afterwards, she'll joke about showing you where young dryads come from, but then she'll get serious and tell you that sexual partners for dryads must have two attributes: they must be strong and agile, and they must be able to procreate.

To prove that you're strong and agile, you'll just need to kill a wolf and give Morenn a Wolf Pelt. You can find enemy wolves just to the west of the druids' grove. For the procreation part, you'll need to convince Morenn that sex without procreation is still beneficial. She'll only believe you if you choose the "sex relieves stress" dialogue option. If you select a different option, then she'll get mad, and you'll have to wait a day before trying again.