The ending was confusing...

first off the movie (and book) is called "the Virgin Suicides" so plural, much like in Romeo and Juliet, you know that they are going to die in the end.
It is necessary for people to have an direct - articulated reason to commit suicide, it is more related to a feeling/overall mood of alienation and not seeing an other way out. You could pose that they committed suicide because they thought they couldn't escape that house - that family - that life...

second, regarding the Virgin question - the notion of virgin doesn't necessarily point out the chastity of a girl/woman - the origins is from a time that every young girl was a virgin/maiden - pure and innocent, not necessarily the fact if they had had sex (although that was the underlying thought - it isn't necessary to be called a virgin to actually still be a virgin)...

I agree, the ending was cryptic. I think that was deliberate. "Virgin" in the context of this movie, I believe, refers to the girls dying before ever really experiencing life...a strange, overbearing set of parental morals attempting to keep their children out of harm's way at all costs, only for those beliefs to be the very weapons that end up killing them. Sad, and quite a moving movie. Why did they want the boys there? I'm not sure either, except that those boys were the only real attention they were getting at that point...an audience for their last act. I liked the imagined car ride; one of the boys (we never know which one the narrator actually is) daydreams about actually getting the sisters out, probably realizing as he's imagining that it isn't going to happen.