This could backfire. David dated Kendra for a while and obviously had sex with her many times. So she could have saved his semen and stored it in the freezer. We know she's batshit crazy. Put his semen in the condom for Alexia to plant in the car. That way it really does look like the affair was real, if it was ever tested.
On my track the issue of semen came up when Karla was giving instructions to "Alexia" and Braxton (?). They asked if they should add semen to the condom and Karla said no because she couldn't fake the DNA results. David and Anna don't know this, of course, so I expected one of them to test it (she is an attorney who should understand evidence, by the way). The results would come back inconclusive, though; they could wonder why no semen is found in a presumably used condom and that could add to reasonable doubt but still wouldn't clear David.
Ultimately, he's faced with proving a negative ("I did not cheat"). It's easy to introduce reasonable doubt. Often you can prove that something did occur. But to prove that something did *not* occur? That's crazy hard to do. His only way out, in my guess, is somehow to uncover the conspiracy. At the moment on my route, Leonard, Chris, and Jake appear to be the only ones in a position to do that. Jake is having doubts about working with Janet. If she isn't able to get the money out of "Alexia's" contact who stole from Darren, this might anger Jake just enough to spill something.
The irony here is that, unwittingly, David's father is funding the destruction of David's marriage and legacy (the family business). That's some clever writing right there...