The problem is, Rusty didn't do it. If you're Rusty, you'd be wondering how your semen got there. Wait a second . . . What if she was so hurt by it . . . Of course, you don't need red herrings to write your mystery.
Clues are seeds.
They plant question and doubt in the reader's mind. The farmer . If you're a confident writer, and you know what you're doing, you can make them obvious and let them make a lot of noise, hoping the reader would think they're a smokescreen, only to be surprised that they are true. Or they can be part of a subplot, like Rusty's marital problems, which actually tie in to the main problem. Or they can be blended in with the false clues, or the facts that seem to matter but don't.