I'm going to give you some very hard, possibly upsetting advice first. Please don't stop reading, because we're going to get the hard shit over and done with and then we'll move on:
Be glad your parents are honest about their disapproval.
Sound like it sucks? Trust me, it does. You know what sucks harder? Being 18 years old, having just come out, having heard the "oh, I love you no matter what, I can't wait for you to have a girlfriend, blah blah blah support blah blah wonderful unique blah" spiel and then three weeks later you come home for a surprise visit from college and your mother is on the phone sobbing to one of your relatives about how she can't believe you'd make such a terrible choice and all she ever wanted was to do what was right for you and how can't you see that you're just confused and you'll get over it. Sucky, right?
Ah, but this is where I hope you're still with me, because here's the lighthearted fluffy sequel: I walked out the front door, down to the street, and a mile away to my grampa's house. He gave me a cup of tea, listened to the whole thing, dried my tears and said "You still have a bedroom here, you know. Let me take your bag back." I lived with him for the next two years. He shared stories about my girlfriend (when I got one) at the dinor during "my grandchild's significant other" conversations. He called me on the phone to tell me he saw something in the paper about the Pride parade and asked if I was going. When I went through a butch dyke period he helped me find guys' shoes in my size and bought me a pair of unisex hoodies I could wear no matter what was under them (they fell apart many years ago from an abundance of love, but I still own the two he replaced them with!). He defended me against ignorant comments when they happened. And he told me, whenever I needed to hear it: "I don't have to understand to know you're happy, and I'm happy for you."
That's right: my biggest supporter when I was a wee little baby gay wasn't my middle-ticket, so-called bisexual hippie mother--it was my Republican-voting 77-year-old GRAMPA. (I'm happy to report that today he's 84 years old, reconsidering his political position and still my biggest supporter--in 2012 he voted on only one thing on the entire ballot, and that was equality for Washington state!) The purpose of this story is twofold:
1) All is not lost because your parents are ignorant. There are other people out there, some of them the least-likely candidates you can imagine, who will love and support you in ways big and small.
2) All is not lost because your parents are ignorant. You can talk sense to ignorant, truthful people, if they're willing to unplug their ears. When it comes to two-faced liars, however, you're screwed. (My relationship with my mom has never recovered, and now there's a gritty reboot that doesn't acknowledge the original: she knows I'm bisexual and just looking for the right man, all evidence to the contrary.)
So how do you make your parents not ignorant? Here's the sad and sucky news: sometimes you can't. Sometimes they not only want to remain ignorant, they will twist your own words into some weird version of "truth" so they can prove that you agree with them. But sometimes?
Sometimes you get lucky.
Other folks have provided a lot of resources for you, but I'm going to add one: http://www.wouldjesusdiscriminate.org/bi…
Click around the left-hand toolbar and you'll learn some incredible stuff that's in the Bible--that Jesus blessed a gay couple; that one of the greatest kings in the Bible was bisexual; that our modern wedding vows are based on the vows, Biblically given, of a lesbian couple; that the early church welcomed a gay man; that Jesus said some people are born gay, and are blessed of heaven! Best of all, this website is run by a church--it's actual pastors and deacons writing these essays. It basically goes through not only the verses used against us, but verses people completely overlook (like the one where David and Jonathan were apparently not only lovers, but married--to each other), to show that far from the Bible condemning gay relationships, it contains within itself some very strong support for them.
Best of luck. Check back with us and let us know how you're doing, okay?