I was recently re-reading my favourite Charles Williams novel The Place of the Lion and noticed for the first time that the lovers (Anthony and Damaris) are first cousins.
In the novel, the fact that they are cousins is treated as a 'good thing' - it seems to imply that there is a kind of familial love between them, as well as an erotic love.
In Tolkien's world, Hobbits often marry their cousins - and this also seems to be regarded as 'a good thing' on the whole.
Yet this matter of cousin marriage (or sex) is in fact highly controversial around the world: ranging from societies where cousins (even double cousins - on both the mother's and father's side) are more-or-less forced to marry, through a mildly pro-cousin marriage atttude such as that exemplified by Charles Williams and Tolkien, though indifference to cousinness as an irrelevance to marriage, to first cousin marriage being actually illegal (in many US states).
This is a quite remarkable - but almost un-remarked - range of opinion.
Anthropologists have shown that some societies more or less depend on cousin marriage for their cohesion (a cohesion based on genetic relatedness - the cohesion of 'clans'); while for other societies - especially more modern societies - the cohesion of families is a direct threat to the cohesion of the larger state.
Of course there are concerns (or pseudo-concerns) about 'in-breeding' being a bad thing; on the other hand there is some evidence that a degree of inbreeding is beneficial to biological fitness.
I simply flag this up as a fascinating but almost-ignored divergent aspect of social morality: is the ideal spouse a cousin, or anything-but-a-cousin?