If you want to know what marriage looked like in Biblical times, that's a question for the history site. Or the Judaism site, though they'd likely prefer those be asked on History.
If you want to know what specific passages of the Bible that talk about marriage mean, that's a question for the hermeneutics site.
If you want to know how specific groups of Christians define "biblical marriage" then that's a question for this site.
However, your question asks the first, and implies the third. That makes it off topic on the first, and too broad on the second. No, protestantism is not narrow enough, different groups of protestants define Biblical marriage differently.
We, as moderators, and as a community, aspire to put questions on hold before they attract answers so that answers match the final state of the question as closely as possible. Your question is either off topic or too broad (you can choose via edits). That means it should be closed pending an edit to correct or deletion.
This does bring up a slightly larger issue, what issues are open to straight up general biblical interpretation, that's slightly out of scope for this meta question and I'm going to leave that alone for now as it needs to be part of a more specific discussion with more community input than we are going to get here.