The pattern of romance is:
1) These two characters should NEVER get together.
2) But, hypothetically, if they did they’d be perfect for each other.
3) Repeat 1 and 2 to hit wordcount.
4) They get together.
It’s surprisingly difficult to come up with reasons characters shouldn’t be together that aren’t impermeable but are valid. These reasons shouldn’t be possible to overcome with an adult conversation about needs and limits. However some change in circumstances, relatively unforeseen, should remove the reason keeping them apart.
The trick is that the easier it is the remove the reason keeping the characters apart, the lower intensity point 1 will be. That reduces the whole meaning and weight of the plot. But if point 1 really is never-ever-ever, than when the two characters come together, the story may not work outside of niche.
Some of this is set in terms of theme. So any remotely realistic story will have some guide-rails. Alice can’t murder Bob’s wife Charlie, and then have a healthy relationship with Bob. That’s a bridge too far. But a non-realistic story might be able to break that rule. Alice can murder Bob’s wife Charlie if Charlie is a vampire. Now we’re in vampire romance, so eh, murder away.
Also I, the writer, have to be able to think about the characters involved in a meaningful way. I’ve got guiderails and so forth. I also don’t really read vampire romance; my appreciation for vampires is mostly about the mythology and thematic elements. This exposes me to a lot of really, really trashy romance fiction, and I get it, I just don’t appreciate it. Some time ago I read Tanith Lee’s Night’s Master. That also fit into the category of ‘I got it, it just didn’t work for me.’ That’s why I described Night’s Master as better than I liked it.
If the characters solve ninety percent of their problems with gun-fights, the readers don’t have a moral high ground to argue trashy romance is bad. But there is a ‘this doesn’t work for me’ argument, which is both infuriating from inside looking out and yet valid.
The point is I’m not writing Alice murders Charlie, vampiress, and gets together with Bob unless… actually, I’m not sure how this would ever work. It’s probably possible, but I don’t see it happening.