[MagdalenaR]

There have been some good suggestions already, especially McMaster Bujold's Vorkosigan books (Shards of Honour, etc.).

Karen Lord's The Best of All Possible Worlds is a great love story as well as scifi.

T. Kingfisher would be another author to look at: I started with Clockwork Boys. She does some off-beat revisions of classic fairytales as well (e.g. Bryony & Roses--Beauty & the Beast).

Sharon Shinn would be yet another to look at. I liked Archangel both for the romance & the setting. This is the start of a series of novels.

Martha Wells, the Murderbot Diaries writer, has some good fantasy novels as well. The romance tends to be a bit more in the background, but The Element of Fire & Death of the Necromancer both have a romance as part of the plot.

There is a series called the Great Space Race where a bunch of writers each wrote an novel/entry in a reality show--these are racier in the other sense of the word too. Teresa Noelle Roberts, one of these authors, has a series loosely inspired by Firefly called Chronicles of the Malcolm. The first one is Thrill-Kinky; it leans toward erotica.

A much better, Firefly-inspired novel with a sweet human/non-human FF romance in it, is Becky Chambers A Long Way to a Small Angry Planet. (A really great & heart-warming novel!)

A bunch of the late Anne McCaffrey's novels had a romance element, e.g. the Dragon/Pern novels as well as the Freedom series, which started with Freedom's Landing. To a lesser extent, so do Seanen McGuire's urban fantasy InCryptid novels.

Two novels with interesting AI/robot-human romances are He, She and It (Marge Piercy--won awards) and The Mad Scientist's Daughter (Cassandra Rose Clarke).

If MM romances are to your taste, K.J. Charles is definitely a great writer. A series with less explicit sex & less of a tilt to horror is Melissa Scott's Astrient novels. The series starts with Point of Hopes, which she co-authored with her late partner Lisa Barnett. C.L. Polk also writes great fantasies with queer romances, starting with Witchmark. G.L. Carriger's The 5th Gender is a sweet alien/human MM love story as well as a scifi mystery. Heather Rose Jones' Alpennia series is a lovely set of alternate history with magic stories that revolve around FF romances.

There are also lots of writers who do more formulaic work. Jayne Castle does romances in scifi settings. This is one of Jayne Ann Krentz's other pen names, so if your wife is a JAK fan, this might appeal.