He shouldn't feel so insecure. The forest was his domain; these were his trees, his rocks, his animals, his men, and this was his woman. She should have been safe here - safe, at least, until that man came.
Guy of Gisborne, the wolf who ravaged Locksley. Put him in a room filled with people and he'd smell out Marian in two heartbeats. His name alone made Robin's skin itch.
How could Robin ever have protected her - here or otherwise? When she was in the castle, surrounded by enemies, snapping at her feet, he was less nervous. He simply imagined those dangers away. She was locked safely away in a tower, he told himself; she was in the town bartering at the market. Gisborne was in another village - never Locksley - or out on sortie. It was a big place, a city, really, and chances were they could go days without meeting. But here, in the forest, her geography was a less existential thing. Her body couldn't float away; it was grounded down to a particular size and shape, with weaknesses he couldn't argue away.
He saw her in the morning at breakfast, at noonday, throughout, and he saw her when they both lay to go to sleep on opposite sides of the hideout, her hand in his across the space between the mats. He took her with him everywhere, hardly even let her walk off for a piss without promising not to go too far, and take a knife. That she obeyed at all was a testament to how hard her father's death must have hit her.
He had her now close to him, and all he could think about was her absence.
So on the day that Gisborne walks into his forest, brandishing steel and fire alike, it's his worst nightmare coming true. Marian begs Robin to let her go; she twists the ring he'd given her about her finger, like jostling with a locked safe.
"I have to go; I'll be safe with Gisborne, and you can get away. I'm more use in the castle."
Her hurried glance and her casual words - safe with Gisborne - strike him in concerted blows. As he kisses her for the last time, before leaving her to the other man, his heart twists so hard that he almost gasps. He knows that she will not come back.