Well the fact that it is not unavoidable, is a bug in this version of the game and it should be avoidable in 1.8b if i understood the writer correct. To be honest I had not gone for the exclusive route in the hope something like this could happen and you would be able to get into a love triangle with Christine and Jess, even if it might be a riskier route. Seems to me that is the only way to keep fooling your sister and survive the end of the game.
So i did enjoy the Christine and Jess scene on my play-through and it did feel kinda natural.
Do agree though about the general remark about Netorare opponents are always douchebags, it likely comes with the kink.
Though a game with NTR where you lose the girls to guys who are actually nicer and better men as the MC if you do not take your chances or go to far to get them could be nice with a whole load of appy ends for the girls you miss out for and lesser or bad ends for the women you manage to catch. Not sure that would be popular though.