Or...MC ends up screwing both of them. To me its kinda obvious that is where it will end up going. *
cough* harem.
Still Idk how you can be a cuck if you hardly know the person in question *and* they had a thing going before you even met them. I'm no fan of ntr or being a cuck for that matter, but that scene hardly registers as such. It's more voyeur than anything else if you ask me...or is that now synonymous with being a cuck? I...suppose it could be taken differently if you are not into girlxgirl stuff, but again MC has hardly said two words to Rhea. She wont even agree to come to MC's room so, plot of being a potential wife aside, that "engaged" really is a stretch.
Being a harem game, lesbian content is probably all but inevitable and if "lesbian cuck" scenes, as you put it, happen it probably wouldnt even register as such to a vast majority. I certainly didnt see it that way, and Joy doesnt strike me as the "dominant screw you in the ass with a giant strap on" type, so not threatening...at least from my perspective.
All the girls tend to end up in your bed in the end so the lines are blurry to say the least.