If it was such an important issue (which it clearly is to some) and as its such an important issue for the LibDems then people have had adequate chance to vote in favour of it already. They didnt.
We vote for packages of policies. People had their chance to vote on this issue. If other issues were more important to them and that meant that they voted for a different party then it shows that this doesnt rank high in the priority of the British public and not worthy of a referendum.
Referendums should be seldom, if ever, used. Certainly not for a policy that, as you both said, was not important enough for people to change their vote over and certainly not for issues where the supporting party is the weakest of the 3 mainstream parties.
Id be in favour of looking at election reform if there were real signs that the system was broken. It isnt PR and to look at the numbers and see a difference between vote and representation ignores the act that it isnt designed to do that.
If the LibDms had a greater proportion of the vote than Labour and got far less seats then Id be willing to listen to arguments for change. That is certainly possible and could illustrate the system has flaws. However, it didnt and essentially, in every election the party that finished 1st in proportion of the vote got the most seats, 2nd got the 2nd most and 3rd got the 3rd most seats and while the hung Parliament right now is not ideal it is superior to what would occur under PR.
EDIT- Just to note, I have never voted in an election or been a member of any political party. Im not saying this in defense of any group.