It can depend on which publisher you target too. There's niche markets inside romance. I'm a dark romance (psych thriller) author; I'm also an editor with Dreamspinner Press with both their DSP lines (romance) and DSPP (no core romance needed). What I write wouldn't be published with DSP or DSPP because of the dark content.
However, I've not read a romance from an antagonist's pov, but I have read a romance element in a psych thriller etc from the antagonist's pov, one beautiful one where the antagonist (a psychopath) falls in love and drives his lover (a sociopath) insane and into prison. It all comes down to the tags you use. The last story, although it has a core romance theme, is still a psych thriller, and tagged as such on Amazon. The MC in that was cruel, devious, an... *expletive deleted*, where I wanted to get inside the novel and gleefully throttle him a number of times, but the counter-play from the sociopath made it a train wreck I couldn't stop reading, and I needed to see how on earth they'd get an HEA.
Other novels from the antagonist's pov is Lord Foul's Bane, where the antagonist is a rapist. That one is a firm no for me: there's no way you can make a rapist sympathetic, I'm sorry. My thoughts are with the victim.
Again, it comes down to the tags you give the novel.
There are multiple dark romance readers out there who would read an antagonist's pov when it comes to a romance element for dark romance, but I think they'd be expecting the antagonist to be the one conflicted or challenged by a lover, not so much as a watcher of another couple (unless it's a dark love triangle!). But again... tags. It will all depend on the tags. Don't run with the romance if the (dark) fantasy is the main draw. E.g., the psych/sociopath romance was tagged as a pure (gay) psychological thriller. No mention of romance.