is she lying or telling the truth?

do you think you should just trust the women when you ask if she would you take a polygraph test about the things she told you about her past like whose shes been with and what not before you marry her.. like asking as a joke but acting like your serious and she says no you should just trust me.. would you trust her or think shes hiding something? i know it sounds weird but i am with a girl i'm engaged to and asked her just as a joke but i was acting serious to her.. cuz she had a bf for 8 yrs but i'm her 3rd guy shes had.. lol supposedly.. so i asked would u take a polygraph test just about who you've been with and other questions. don't get me wrong trust isn't an issue and i know she'd never cheat.. i just think she scares to tell me the truth about it thinking i would leave her just cuz she slept with another guy/// i'm not a weirdo n def not controlling i just was thinking about it and thought id ask.. Give me your opinions ..