To avoid moral complications, which are sure to turn any argument nasty, I will be purely factual in my statement.

The official definition of murder is "the unlawful premeditated killing of one human being by another."

While abortion is premeditated, as long as it is done in an area where it is lawful, it isn't murder.

Because children are not fully developed mentally, it's much easier for adults to manipulate and coerce them into engaging in intercourse. Studies have also shown that underdeveloped minds are more likely to engage in risky scenarios and keeping that in mind in combination with the lack of readily available information for minors regarding sex, one can see how allowing minors to engage in sexual activity could allow for disaster.