Having a wide range of options is usually what I prefer since things change, especially in text-based games where you really don’t get the same immersive exposure to NPCs, so I’d like the opportunity to later on - maybe if the pacing is unfavourable or I’ve found out something unsavoury about them - to change attitude towards them, rather than being locked into a certain path of behaviour towards them. Also because things really do depend on situations, so having a wider variety of choices allows for flexibility. A responsive NPC whose reactions depend on previous choices would also be preferable, especially if you behave erratically and they try to figure you out. I vaguely remember one game being like that, but not sure if it was a CS one.
As for second part, I honestly feel that few stories allow the MC to flirt from the first meeting (what you describe as “unrealistic” probably applies more to an actual romantic relationship, but for more casual ones I don’t find it unrealistic), or to otherwise choose not so much a romance but a fling, the dynamics of which are then affected by and affects whatever’s happening in the main storyline. (Is a fling partner or friend with benefits still referred to as an RO?) Been on the lookout for one of those “oh hey, we’re supposed to work together now but all I remember from last night is you and too many shots of vodka” moments - would certainly add an enjoyable tinge to a story’s atmosphere, but I haven’t come across any.
Situations where you have to always chat with the RO and find out their past and have them pour out their emotions to you all the time just to raise relationship stat so you can do a little hand-holding later on are frankly exhausting and I usually stop reading those, especially if romance is a huge part of the story. So basically aromantic or less emotional ‘romances’, but it’s understandable that with some plots and characters that’s not exactly feasible. From what I’ve seen so far it’s a minority preference anyway.