If 11 and 12 year old girls are to avoid having sex, it's up to their parents [and older sisters, aunts, if any] to make them understand why. But all parents aren't good ones, and even some good ones can't/won't talk about sex, and all kids don't listen anyhow.
We don't want them having sex until they ready, [different ages for everyone], for a lot of reasons, but the sad fact is that some still do. Even at age 11 and 12. Pretending it doesn't/won't/can't happen just leaves them to deal with STDs, or pregnancy, [or both], and how many feel comfortable telling the parents about that? How many babies are abandoned at birth, because a too young girl couldn't deal with it, and/or was terrified to tell Mom and Dad? [If there is a Dad - they're not always the most responsible parent of the two].
Keep pretending that all parents are responsible about informing their offspring about the reality of sex, and all teens [and preteens] will behave with good sense [when their hormones are raging] and we'll keep having 11 and 12 year olds having babies, and outbreaks of chlamydia in middle school, like the one that just hit Texas.
Keep pretending it's not what it is, because that's worked well so far, right?