If you convince Liz she is being ridiculous, you are insulting her. Calling anyone ridiculous is an insult.

You should apologize to her directly for not telling her of your plans, but tell her you did not know she had feelings for you. What you don't need to add are these things: 1. She is fat and her health makes you think she would not be a good mate. 2. You're afraid she would colonize you. 3. You deserve to have a wife as good-looking as you.