I am not a fan of cheating, can someone clarify something for me? The game starts with he protagonist apparently having a GF, but he moved away, so I guess long distance relationship? I don't really feel like playing if she is not in the game while protagonist would be cheating if he gets with other girls. Does she breakup with him or cheats on him early on setting him free? Or maybe she is one of those cuckqueens that is chill with him getting together with other girls?
Did you read the changelog on OP? This new update is the first time we actually get to see the old GF and the ingame test explains all about it. You as MC get to decide what to do with it all, just takes some carefull reading and saves at the right times in case you want to change a decision/outcome.
The ingame guide on what to do next, to reach content, is pretty accurate and good, so the game isn't that complicated, can appear a bit grindy at first, but has a more realistic feel to it.. for example getting and keeping fit, as fitness level decrease slightly if you don't train any more.. and fitness level is needed for certain scenes.. so you have to find a balance during your days of the week for everything and that's the main challenge as a player. When to train, work, date, etc etc...
If you go for achievements, there's 2 different for the MC parrent's, one for calling them a lot for "help" which is a free boost early on.. and one for not calling and hence be grown up and independend. You can get both as I recall, by first using the phone for extra cash and energy boost many days in a row, and after story is set up to earn ya own keep, stop asking for their help from then on.. this is the time where finding your game balance becomes more challenging
As for your cheating question: You get to decide what you feel comfortable with and the consequences of such decisions should also feel rather logical. It is a "porn" game after all and most players would like a Harem, but it is possible to be "the good guy" and miss out on harem options, hence the "save" before big decisions.
As for NTR concerns: So far I've not spotted any NTR with other males. Some girls fool around with other girls, but that can just lead to the MC to fool around with both and get 3somes, which I think most players would just find extra hot!
Edit: As far as I understand it, regaring the "ex-girlfriend" MC thought she broke up, she thought it was just a break, she shows up, wan't to get back togther, find out he is seeing others, wants a chance, get's the chance if MC wants to. MC get's to decide what to do with it all.
She isn't a Cockqueen who is cool with him sleeping around with everything that moves, most of the females arn't, but this game gives the option to do it anyways and that's part of the fun.. for some/most of us anyways
Hope this answers it all for ya?