Because Alvaro knows that if he is the one who broke her heart instead, he would never get over it. Alvaro is terrified that he will make a mistake and hurt her somehow, and he couldn't bear the possibility of losing her. Luna is his happiness, his everything, and he doesn't want to lose her. He knows she has her fears about being a mom, about having kids, about her being an omega, but Alvaro is determined to make sure she has children and she won't worry about any of those things again.

That's why he asked her to marry him, because he wants her to experience those feelings and...