If you are sticking to heterosexual romances that varied a bit from the norm:
Mata Hari (1931)... with Greta Garbo and Ramon Novarro. Oh boy! This is an odd one, but I love how she bravely walks to her death sentence without telling him, since he is blind and thinks he is visiting her at a sanatorium. I love Ramon in all of his movies, because he behaves like such a wide-eyed innocent.
The Hurricane (1937)... poor John Hall goes through ssssoooo much torture and adventure just to be reunited with Dorothy Lamour. It takes a while before Mary Astor manages to persuade Raymond Massey to just give up trying to keep them apart.
Now Voyager (1942), the old war horse we've all seen many times with Betty Davis' love for Paul Henreid later directed towards his daughter, resembling her own self as a youth.
The Band Wagon (1953)... sure, it is a musical but I love how "museum piece" Fred Astaire finally wins over Cyd Charisse. Also her speech at the end of the movie about kissing her obstacles goodbye... or whatever it was she said.
Angst Essen Seele Auf (Ali: Fear Eats The Soul) (1974) involving a couple a few decades apart and vastly different racial/cultural backgrounds. The differences in their lives break them apart, but not without trying. The male lead was a lover of the director and his real life is an interesting, but tragic, tale in itself.
Howard's End (1992)... I recently re-watched it the eighth or ninth time (since first seeing it in a theater) and it consistently gets better with each viewing. Several romances here to comment on. Margaret Schlegel (Emma Thompson) has a very close friendship with Mrs. Wilcox (Vanessa Redgrave) that carries through her selfish husband (Anthony Hopkins) who still needs her in the finale for emotional support, especially after his son gets into trouble for indirectly killing the father (Samuel West as Leonard) of Margaret's sister's (Helena Bonham Carter) child (and that is another love triangle of sorts involving his wife Jackie). I especially love the scene when Margaret first observes the Wilcoxes from her flat window while critiquing brother Tibby for not being the proper host. She sees a tenderness in both Wilcoxes that she repeats after she marries him after her passing.
If you want to break away from the "heteronormal" (a word I purposely put in quotes only because many in this country are still not completely accepting of anything that is not):
Weekend (2011) directed by Andrew Haigh. I was debating whether to mention this one in discussions of David Lean's Brief Encounter and Summertime. Key difference is that nobody is married here and cheating on anybody, yet the romance is still short-lived and we still see the two guys say goodbye at a train station (a very David Lean-ish scene). One of the stars was heterosexual (with a girlfriend) and the love scenes were allegedly filmed by a woman, probably to make him more comfortable. There is some nudity and very provocative scenes, but not nearly as much as most heterosexual romances we are used to. I could easily see TCM showing it late at night after the kiddies go to bed.