Even Scandinavian countries these days would sooner go republican than enter a personal union. I can't see Danes, Norwegians or Swedes passing such a matter in a referendum.
But why would the issue even come up in a referendum?
Basically it's nobody's business if two monarchs happen to marry unless that marriage violates some law.
In any case, for Britain and the other Commonwealth nations, other countries monarchs (and their descendants) are already in the line of succession (King Harald V of Norway, King Carl XVI of Sweden, Queen Margrethe II of Denmark and Queen Beatrix of the Netherlands).
For other countries the line of succession is more limited (e.g. Dutch line of succession is limited to those within 3 degrees of kinship to the current monarch), but nothing in the laws states that the monarch should be monarch of that country only (although there are provisions in the Swedish act of succession that the monarch must be brought up in the Church of Sweden and that any heir to the throne cannot marry without the consent of the monarch, nor can any heir become monarch in another country (by election or marriage) without the consent of the monarch and the government).
So a situation could arise where say...Prince Charles dies before inheriting the throne and then Prince William eventually becomes King, but he doesn't have any kids (or his kids die) and Prince Henry marries Princess Madeleine (3rd in line to the Swedish throne and the marriage is approved since nobody expects Madeleine to really be in the running for the throne) and they have children (who are brought up in the Church of Sweden). Then say all the others in line to the Swedish throne die before producing children or are found to be sterile.
At that point any number of scenarios could arise:
1. Prince Henry remains a Prince but Madeleine becomes Queen of Sweden and their first child will be in line to both the British and Swedish thrones.
2. Prince Henry could become King of Britain and Nothern Ireland and Madeleine remains a Princess but their first child will be in line to both thrones.
3. The both become monarchs in their respective countries and their first child is in line to both thrones.
4. Neither becomes a monarch but their first child becomes King of Sweden and then King of Great Britain and Northern Ireland.
I'm sure others could come up with a much more complicated scenario in which this new King of Sweden and King of Britain and Northern Ireland would have children who end up getting married to heirs to the Dutch throne and where the Danish throne and Norwegian throne are already in personal union through a similar Henry-Madeleine scenario and then the final personal unification occurs when an heir to the Swedish-British-Dutch thrones marries an heir to the Danish-Norwegian thrones and produce a child that would be legally entitled to inherit all 5 thrones.