Yeah, I'm not too sure. I went to Catholic school but am not Catholic. I think it is the premarital sex. No one is going to admit to it, but when you get knocked up it's like getting caught red handed. I guess the sin would be to continue to have premarital sex with the pregnancy? I don't know. I just know these girls were having sex and still taking communion, which I was under the impression was also a sin.
I also agree with your last sentence. Hypocrisy at it's best. (that goes for all religions)