Many pupils will have strong views pro or anti-marriage, largely based on what has been role-modelled for them, for better or worse. Therefore debate can easily consist of assertive and sterile monologues. So National RE Framework attitude target "open-mindedness", defined as "being willing to learn and gain new understanding" is crucial here (NF, p13).
Use dictionary searches, on or off-line, to define "common law" and "legalised" marriages. Research how partners are "found" (is the dating agency just a higher-tech equivalent of the arranged marriage?), then brainstorm why some marriages succeed and what might constitute success in this context - more unusual than listing why marriages fail. Contrast what sacred texts and media agony columnists have to say about marriage and marriage problems. Survey anonymously what pupils themselves hope for and why.
Explore how any relationship can (or can't) protect the partners involved and the children, if any.