The first thing that comes to mind after reading this article is that they don't have enough evidence to conclude that the fact that there were sperm in the victim that weren't from her boyfriend actually means that they were murdered by the creator of said sperm. It seems likely, but calling the case 'solved' just based on that is a bit of a stretch to me. Maybe the article is leaving out some important details.
Come on, we all know that any woman who cheats on her boyfriend is immediately murdered by the same guy she cheated with.
We don't! We just assume!