Plato and the rules for marriage
Plato's regulations on marriage principally apply to the guardian class, the philosopher-rulers. We need to distinguish two senses of marriage in the Republic. These senses are not to be distinguished by specific terminology but by the natural sense of words in context.
In the first place, the active guardians, who attain guardian status only on maturity, marry solely in the sense of forming brief sexual unions (Rep.V.460b), regulated by the mysterious 'nuptial number' (Rep.VIII.545d-547a) - which, incidentally is eventually miscalculated and results in the downfall of the kallipolis, the ideal polis or community, as described in Republic, VIII-IX. Call such unions, necessary for the continuation of the kallipolis and of the high calibre of reproduction of the guardian class, 'M1' marriages as they may be termed. As merely copulatory they are not different from animal matings.
Marriage and the older, ex-guardians
Only when the guardians, the philosopher kings and queens, are well past maturity and no longer able to rule or to procreate are they granted the pleasures of marriage - primarily the pleasures of one to one companionship (Republic, V.461b-c). These are 'M2' marriages, as I'll call them. For such marriages Plato sets constraints against incest and if sex, in the great majority of cases unprocreative, should happen to produce pregnancy, 'then ideally the offspring should never see the light of day' (Republic, V.461c; The Republic, tr. T. Griffith, Cambridge: CUP, 2000: 159).
I don't see how it can reasonably be denied that Plato recognises marriage as pure copulatory union and marriage as companionship for the superannuated guardians. I have referenced the distinction. It does not follow, because Plato uses the single term gamos for marriage, that therefore the term has the same sense whenever he uses it. Context etches a clear distinction. If I am blindsided on this point, I will of course retract.
To take a lead from Terence Irwin, temperance involves an agreement or harmony between the different parts of the soul (psuche) such that:
temperate people's appetites are not strong or wayward enough to cause them to act contrary to their true beliefs about what is good and bad. [It is] an orderly condition of the appetites. T. Irwin, Plato's Ethics, Oxford: OUP, 1995: 226.)
In the case of the guardians, this orderly condition will not be a matter of chance but of the education outlined in Republic, IV-VI. And in light of that education it might be better to talk of knowledge rather than of true beliefs. The guardians, young and old, know the good.
I cannot see that post-retirement marriage [M2] violates temperance. The elderly guardians can no longer help the polis and conduct its government. Marriage 'serves no obvious purpose to the state' but then, there is no obvious purpose of the state that the elderly guardians can fulfil; their day is done, their work is over.
Marriage may not be a 'necessity for men' in old age - and don't forget the retired philosopher queens - but it is also harmless. The elderly are free to have sex with one another and to form permanent liaisons. Freeing up this largely impotent aspect of their sexual lives, previously so strictly controlled, hardly entails a distintegration of their moral persponalities in respect of temperance or any of the other virtues.