HOW DO I TELL MY PARENTS I'M BI?

i'm bi and i need to tell my dad and stepmom. i no my parents don't staight up hate gay people, but i don't know really know where they stand or how they would react if it was their daughter. I have a girlfriend and we want to get married so I'm gonna have to tell them, but I am so scared about their reaction. I don't want them to think biuality is a phase or something. My parents are divorced and i told my mom and she thinks its just i can't get any guys and i found a friend who i'm close to so i think its love. She is SOOOO wrong though. I don't know what to do! My mom doesn't take it seriously at all! She says because I haven't been in a serious relationship with a guy, I've found a close friend who is a girl and I'm lonely so I'm mistaking close friendship for love! How can I convince her otherwise! I KNOW I am madly in love with my girlfriend and I want to spend the rest of my life with her. I've never been as happy as I am now that I'm with her. I think it would actually be easier if I was coming out as a lebian rather than bi. HELP!!