That's only the haters screaming. The kiss isn't the only reason but a catalyst. Their relationship was already strained when the story started and continued to become strained as the story progressed and some other external factors contributed to the cheating. If you are someone who loves a good story than just the porn then you won't regret it. This VN's foundation is the plot rather than anything else.
Thanks to those who replied. I'll probably give it a go when I get the time (at least til the cheating part to see if IMO it feels right). Seems to me there are a few plot holes already. I'm guessing the whole note from dev about "winning" has to be about saving the world and possibly getting revenge on the ones doing the manipulating. From what I've been reading it seems almost everyone is at least trying to manipulate the couple. So by my reckoning stopping the bad guys, getting revenge and walking away from everyone involved seems to be the main "win" path. I can only see saving the marriage as the cuck route(to me that's not an option). Once trust is betrayed there is nothing left to base any personal relationship between the couple on. If Anne is trying to split them up she is just as bad as X (someone who can't be trusted). It will be a while before I can get to play this unless I jump it above a bunch of others so maybe if any other updates come up it may change my mind.
I see alot of talk and speculation about who X can be. Going only by what I have seen here on the forums unless it is someone not yet introduced I would have to go with Leon. Despite the call early on, he could have had someone else make the call to throw suspicion off of himself and to make him look like a pawn. For those who say his family already got what they wanted from Alan, he still wants his wife and/or to keep his life in ruins. Anne could possibly be his "hidden" sister and they could be working together. Which would go even further into ruining the marriage aspect. A group working together and using only a single name to throw agencies off their trail is possible and makes it easier to play the pawn part. The messages from X could be them telling each other key info to further their plans. Leon knowing and telling Anne when to open the door for the kiss to be seen. Anne wanting the details in a diary for evidence to show Alan. Not sure how many of you saw the original Scream movie but if you have you can see why more than one would work better in throwing others off their trail and keep them looking for the "mastermind". Honestly it's probably too early in the story to really tell. I'm just throwing out a hypothesis that nobody seems to have mentioned yet. Again I have not played any of it yet this is just from what I have read here on the forums.
To go with another option X could also be Clara's boss wanting her to actually be one of the best field agents and not retire to the family life or whatever was supposed to happen after this mission. I mean that's one of the biggest plot holes I have so far is how many field agents/spies/assassins have ever had a family or significant other? It's a liability for any operative in the field much less a "top field agent".