"Das Mädchen sieht die Jungen."

[Max2013]

why is the answer "The girl is seeing the boys" wrong?

[Hohenems]

It shouldn't be. It is an awkward sentence, not to mention it may lead the reader to think the girl is dating several boys, but I believe it is grammatically correct.

[helenvee]

Yes it is incorrect. I think it was Christian who gave this link http://esl.about.com/od/grammarintermediate/a/noncontinuous.htm previously which gives a very good explanation.