The authors point out that girls who decide to participate in sports may be significantly different from the typical girl who does not participate. They are likely more confidence, for example. We observe that sports participants are less likely to get pregnant; this is different from saying that a girl who plays sports is less likely to get pregnant than the same girl if she was not on a sports team.
It turns out that sports participation actually increases the pregnancy rate among girls. To show this, the study looks at the effect of Title XI, and the introduction of additional female sports participation, on pregnancy rates. From the paper's abstract:
We find that a 10 percentage point increase in the fraction of girls playing sports in a state increases the teen birth rate by 0.3 percentage points (about a 10% increase). However, there are racial differences in the effect of sports participation. The increase in the teen birth rate is most pronounced for white young women with some suggestive evidence that sports decreases teen birth rates among black young women.Download it here.