Consider Phlebas comes to mind

Most readers agree that the MC of the book is not a good person. He's fighting a war on the side of the Religious Authoritarian genocidal slave drivers, just because he doesn't like the fact that the other side (who are fighting to stop the whole galaxy from being conquered by these asshats) is essentially run by it's AIs. And he goes to extreme lengths to continue to support his shitty masters, betraying people who come to trust him, and just generally being a bad person. It's an interesting read, because the whole book is told from his own perspective, so the fact that he's a terrible person isn't really spelled out for you, all his actions are presented (in his own mind) as totally understandable, and even heroic.

There is a romantic subplot (two, technically), but it's definetely not the focus, and can honestly be ignored for the most part if you're not into it