In Ancient Greece, marriage was the 'transfer' of a woman from her guardian, kyrios, to another man in order to procreate and transfer property. Most city-states, or poleis, have not left us with much of their legislation regarding this type of exchange and Athens provides us with the richest sources.

One important part of Athenian legislature was Pericles' Citizenship Law of 451/0 BC which gives us some insight into how intermarriage between citizens and non-citizens or 'aliens' was perceived in a large and prominent polis like Athens. It actually forbade the marriage of citizens to non-citizens, and children of these partnerships were not to be granted citizenship (which was a prestigious and sought-after status especially under Pericles' laws). This shows that the perception of inter-marriage was not particularly positive, although it must have been fairly common due to these restrictions being enforced.

Just to add, marriages between residents of poleis like Athens and Sparta would have been relatively unusual due to the self-contained nature of city-states and the fact that politically the two were almost always at each others' throats.

Sources and further reading:

• Plutarch. Life of Pericles (37.2 specifically deals with the marriage of citizens and non-citizens).

• Aristotle. The Athenian Constitution 26.

• The Oxford Classical Dictonary additionally has three very well-written, concise and researched entries on marriage in the ancient world.