You loved Draco Malfoy. You had two of his children. But Draco was killed. Sentenced to death after the war. It’s been years without physical affection and your kids are attending hogwarts and your body is lonely. So you turn to Mattheo. He knows you can’t ever love him. Still he lets you use him as you need. And he falls. He falls in love with you. Asks you to physically be his even if you can not emotionally be his.