How do you know when to tell your husband to cumin you?

Last night me and my husband were having sex and he at the end said do you want me to jizz in you, and I answered no because it sounded so dirty, we been married for 3 years we been using the pull out method for 9 years he's never cumed ine before I guess I'm nervous we will get pregant right away we both are 32 years old