The risks outside were still there, but he felt they would be less than Ma Ling would face when she joined the Order. Though he felt slightly guilty about it, borrowing Ma Ling's sympathy for his current state, at least out there he would be assured she wouldn't leave his side.
Out there they would have no one else to rely on but each other, and with Ma Yuan's crippled cultivation base, he believed Ma Ling no matter what ambitions on desires she had, she would not easily leave his side in the state he was in. At least for as long as he did, it was a guarantee that she would always be near him looking after him as much as he was looking after her.
But if they joined the Order, then she wouldn't have to do all that as the Order could ensure his safety freeing her to do whatever she pleased which would no doubt mean throwing herself to the frontlines of whatever chaos the Order assigned to her.