right, if we went back the better part of a thousand years ago, 'romance' as you'd find in the romantic era of writing is different than 'romance' today.
i think of space opera like being a soap opera in space, with wild plots, bad dialogue, and nothing there aspiring to be more than pure entertainment. soap opera: 'it wasn't me who had an affair with your husband and gave birth to his children, it was my evil twin! besides, i... i used to be a man! that was before the plane crash, though, when-- when *i* was my sister's father! and i am my own mother! damn you, test tubes, damn you!' space opera: 'luke, *i* am your father.'
nothing wrong with space opera, of course, it just doesn't exactly reach for the stars in terms of having any deeper meaning. throwing SF into romance, well, while it's not my thing, i think you don't see a lot of it for obvious reasons. i'm sure there's a niche market for it, and romance readers will read anything with a love story just about, but i don't envision the mesh of these two resulting in 'hard' science emergin, i.e. no long-winded explanations about how the bock IV sub-warp engines function in an ion field, know what i mean? and i reckon there are people out there who figger a romance between two divergent species will work, but i doubt you'd find many romance readers interested in that. i reckon romance sticks with the present and history because the readers can relate to those times and not so much with a heroine with eight boobs and thorns covering her body.