Slavery is rarely a voluntary, desirable status. If a family with a slave moved to Europe, the slave would automatically attain equal legal status with the family, and could stay or leave as she wished.Would you make the same argument for someone who became a slave at an early age?
And where does this leave the individuals involved? Wouldn't a working social partnership be valuable in integrating into a new culture?In which case, if the "husband" is honorable, he would have the marriage annulled at the border of his new country.
There are situations where there are no good options. If neither partner is being exploited and both are happy with the situation, might it not be better for all concerned to leave things as they are and let them face the problem of integration as a team?