Friend who is too affectionante with boyfriend!! Awkward!! They wont stop!?

Ok, so 2 of my best friends hooked up around 3 months ago, At first I was so happy for them, but I have grown to hate them now. The last 3 months have been my living hell. I catch a bus to school with them. Because we all get on at the first stop, it's just us and another Friend the majority of the way. They Always sit the seat behind us, this way i don't have to watch them. I just want to get this straight, Im not jealous or anti- happiness or anything, its just super uncomfortable. She sits on his lap the whole hour to school, making out, he's feeling her up, they're hugging, nuzzling... ugh, whilst im sitting their with them. They sit in the back seat, so no one has to watch them now. I asked them on several occasions to tone it down, and loads of times to just STOP! but they don't listen!! When the other kids, get on the bus later, they are really uncomfortable too, they all talk about them behind their back. When they weren't on, no one would sit in the back seat, because its 'tainted'. Everyone discussed how inappropriate it is, and I joined in. They're my best Friends, and Ive grown accustomed to third wheeling, and i feel really guilty talking about them behind their backs. I try putting my ipod in, everything, because they wont Stop!! several other people have told them to as well, they just wont listen. There is a such thing as too adorable and 'A time and a place'. The bus and in public isn't the correct environment I believe to be so affectionate. Any one had a similar problem?? Anyone know a solution?? Please Help!!