Have to disagree on certain parts of your post! Especially what paths make sense, since you omit the fact that the MC can stand up for our couple and does NOT have to be the beta you seem to see him only.
The romance paths (without and with a serious rival) are selfexplaining. Our MC can pursue Mei to make their greencard marriage a real one and you have the option to keep the rival away or beat him/being better than him. That definitely makes sense.
As an aside, our couple does experience with what they like in bed on romance and happy sharing/swinging, see e.g. the exhibitionist trait they discover together.
As for the happy sharing/swinging paths, it is mentioned in game that "sharing/swinging" is a fetish for the MC and our couple goes a way of little steps towards it. It is not that Mei just jumps into bed with Jamal/Davis directly, they have to go a way. In addition, even if the MC gives Mei the option towards sharing/swinging, you do not need to do any of it. e.g. You can tell Mei she can go on a date with Jamal, but you can also tell her not to. Even if you went on the sharing paths, our couple can dabble (only take sharing options now and then), walk towards full sharing or leave it as a pure, not followed, option (does not take further sharing options when they are there). You also can play the MC as a selfassured and able guy. So the Stag/Vixen happy sharing paths are quite nice and make sense.
I give it to you that it is easier with Kati/Jamal, as on their paths Mei in general develops a more funloving, perky personality and everything is more in the open and on the table when it comes to sharing. You and Jamal can also become buddies over time (if you have the needed stats/personality) and Jamal for example likes it when you send the would be assaulter of Mei packing one way or the other. These paths make absolutely sense, esp. in a porn game.
On the Lee/Davis side I agree with you that they make classical sense only if you play a MC with some backbone and ability. Lee and Davis like backroom dealings, D/s etc. a bit too much for a rather meek (low stats) MC to see land for long. Here it makes doubly sense to have some backbone if you want to befriend Davis. But for a porn universe it still makes sense.
I am not a NTR player, doing the NTR paths only for completeness reasons on speedrun, so i cannot say all that much about how much sense these make. But the NTR paths are for the fans of this sexual practice and if they are happy too, it is okay in my book. I agree with you that there are some WTF moves esp. in the hardcore mode, but it was made in specific for the NTR fans, because it forces NTR no matter what. There are less problems on the more normal hard NTR paths, where you can loose Mei quite easily and you can see the stats dropping like stone.
