In theory his personal life is just that- personal- and his situation is perfectly normal nowadays. Marriage might well important building block of society but each couple have their own reasons for having whatever arrangements they prefer and it would certainly be unedifying if prospective leaders started getting married in order to benefit their prime ministerial aspirations.
However given the focus on the leaders' wives during the election it is unrealistic to expect his private life to remain off limits. Back in 2000 there was a leaked memo from Tony Blair in which the former PM expressed his astonishment that he could be viewed as hostile to marriage:
It is bizarre that any government I lead should be seen as anti-family.In other words he grasped that people view the party's committment to family issues through the private lives of their leaders. It is unfair but there you go. Whilst Ed Miliband's situation is really quite common I suspect that if the subject of marriage comes up- for example when the tax system is adjusted to take account of it- any criticism that he makes will be seen as an attack on traditional values and the institution of marriage itself.