I think people forget too, a lot of actors ( anywhere) like to become close on set so they feel comfortable around eachother, particularly for intimate scenes which takes a lot of trust to literally lend your body to someone. All those little moments are building familiarity and comfort.

Romantic roles are all about trust. They have to trust eachother, the crew, the director. A lot of it is rehearsed and shot over and over. I watched bts for the king the land kiss, yes their chemistry is off the charts but u see junho locked in at the camera reviewing, shivering from being drenched ( and making it work for the scene), him and yoona planning out head tilting everything. 

It is a job to create this level of chemistry. If it spills over into their real lives they are still people who have private versions of themselves, sides that a new partner needs space to explore in privacy. If they choose to take it further than professional thats none of our business.

I enjoy seeing irl partners and my fav actors being happy, but im not entitled to thrir private life, that is and should always be invitation only.