Lol, fair, but for this couple I don't think I'd want a *true* threesome OR gyuwon/sungjoon having sex because it'd be so out of character and I love the main couple together too much, plus their loyalty is a big thing for me... BUT.
BUT!!!
I would eat the SHIT out of some more voyeurism play with Sungjoon, with him being in the room *watching* them and desperately wanting to join in but not being allowed and just jerking off alone in his chair as he ogled the main couple having sex (๑•ㅂ•)و✧. MASSIVE bonus points if Hwayoung is taunting him and showing Gyuwon off, all the while rubbing it in Sungjoon's face that he'll never be allowed to touch either of them ლ(´ڡ`ლ).
*ahem*
I maaaaaay have dedicated some thought to this, lol
I'm just saying that sungjoon and gyuwon need to have some dirty nasty sex as a lil treat is all, maybe a threesome if we are feeling it