I wouldn't worry about it TOO much. The option to Romance Sera was still there even when <Did something with Mages, spoiler>.
I got around to romancing Cullen and we didn't agree on a number of things. I believe you can convince Sera to see your point of view, which is currently whats going on with me and if you do her quests and talk to her in between missions for approval you'll be all set. Plus there are a number of side quests where she also gains approval (Just help poor people, she likes that)
When I got to <Spoiler> she was still romanceable to me despite doing something she absolutely didn't really agree with. You'll be okay.
I got around to romancing Cullen and we didn't agree on a number of things. I believe you can convince Sera to see your point of view, which is currently whats going on with me and if you do her quests and talk to her in between missions for approval you'll be all set. Plus there are a number of side quests where she also gains approval (Just help poor people, she likes that)
When I got to <Spoiler> she was still romanceable to me despite doing something she absolutely didn't really agree with. You'll be okay.