Relationships and Liberty
J. S. Mill argues that individuals should be able to engage in 'experiments in living' free from social pressures (provided that they do not harm others). His On Liberty contains only a brief discussion of Mormon polygamy, which he makes plain that he dislikes but is prepared to tolerate provided that it is consensual. The Subjection of Women has much more to say about marriage, unsurprisingly, but I think it fair to say that it's two main themes are: a) Women should have other options in life than marriage, to ensure that if they choose marriage they do so freely (which, Mill assumes, will make marriage more attractive) and b) Partners within a marriage should be able to decide for themselves how it will operate, which may mean dividing conjugal roles, reversing traditional patterns (e.g. the man being a home-maker), or indeed following them.
As far as I'm aware, Mill has surprisingly little to say about alternatives to marriage. (Surprising, I suggest, because although he was writing in the context of Victorian England, his own relationship with Harriet Taylor must have raised the issue in his mind.) This interesting post about the 'relationship escalator' seemed to illustrate Mill's views about the dangers of social conformism quite nicely. Society expects relationships to follow a certain pattern that simply does not work well (or at all) for those with 'deviant' preferences (polyamory, etc), who find their chosen way of life stigmatised.
Speaking of such things, it's interesting to hear tht mixed-sex couples are now appealing for the right to civil partnerships in the name of equality. This seems like a sensible move to me - it could have been avoided had gay marriage been introduced instead of civil partnerships, but it seems discriminatory if homosexual couples have the option of civil partnership or marriage, while mixed-sex couple can only marry. I doubt that it's likely to lead to state recognition of other non-standard romantic relationships though.