Jake is the only character whose sex with Ana can reveal Kendra's plan. If David catch Ana having sex with anyone else like his friends, Hank, Grant or anyone...Kendra didn't set them up...David can catch Ana with anyone of those and get separated ...Kendra doesnt need to bother collecting evidence to show it to David that Anna is cheating...she will remain free without getting caught and David/Anna will get separated..
And even if David catch Kendra that set him up from anyone whether investigator or through Alexia, it doesn't make sense if he catch Anna doing sex with lets say Chris. There is no need to prove that Kendra is behind all these thigns... They will get separated anyways in that scenario..Just an example if David has proof that he's innocent after meeting Alexia, then why the Fuck Ana's doing sex with Chris/Rob/Hank/Grant/Leonard? Kendra didn't set them up. What excuse Anna will give to David that he will forgive her? Anna has to explain to david that she's doing sex with them to take revenge on David lol? David will forgive Kendra in that case for opening his eyes rather than Anna.

And he will marry kendra after that.
So like it or not Jake, Marvin or Chip and Dale sex with Anna needed to be get caught by David to prove Kendra is behind all these. As of now David doesn't know Marvin, Chip and Dale exist and I don't think he will catch Ana with them. So that's how Jake role is important for the story .... his sex with Ana can reveal that everything is a part of Kendra's plan.