Why is Morris Zapp so hesitant to accept Fulvia's advances?

Although Morris finds Fulvia attractive, he is somewhat taken aback by the brazen way she propositions him. He also recognizes that she has read his ex-wife's book, which includes some detailed sexual scenes that she believes are completely autobiographical and he protests are not--thus, he believes she may be disappointed in him after comparing him to the book. In addition, he is "out of practice."