I guess we all have different definitions of what makes an 'intense romance drama'. Some of the suggestions here I do not agree are intense at all. For eg, Boss & Me ... how is Boss & Me intense? It's more of a cute, light rich boy falls for poor girl romcom. Very sweet and great to watch (I loved it!), but not particularly intense in my opinion.
An intense romance for me is where you can feel the raw passion, chemistry and sexual energy between the leads. They love hard and are not afraid to express their lust and love for one another in a way that literally pops off the screen. The dramas I rec'ed above are like this, but dramaland is sadly less inclined to these type of dramas and tend to go for cute, silly romcoms.
I'd say if romance was the focus of Cruel City, it could definitely be defined as an intense romance ... but the romance was only a very small side dish to the main course of the plot, so any intensity we got was very brief and non-recurring.
Cruel Romance was an okay watch, somewhat passionate, but not to the point where I'd describe it as intense. Same goes for Que Sera Sera. They were both good, mature dramas that are well worth the watch, but if anyone's looking for an 'intense romance' as I've described it above, these dramas -- and most of the drama recs here -- don't fit the bill.