The single most appropriate answer isn't there and has nothing to do with the sex necessarily being interracial:
it's when the marriage is on the rocks to begin with.
But that was not the question. While I agree with the sentiment, my question was when does 'inter-racial sex' is 'most likely' to break up a marriage? Not any other issue as other issues are perhaps not relevant to the theme of this site.
In Zimbabwe I knew a couple who married as soon as the husband completed university. He was recruited 'from university' by an African businessman who offered him a better and more senior appointment than he could expect in the normal UK career path.
Hos long term girlfriend, from a small yorkshire village, 'persuaded' him to marry her and take her with him rather than her waiting for his return. They married and flew from 'home' to 'honeymoon' to 'Zimbabwe.
There was no underlying problem in their relationship.
His African boss and his friends took one look at his wife and targeted her. He was sent running up and down the country solving the 'problems' he had been employed to solve. He was new to work, new to the issues, eager to please and show his worth, and experience the new country and the new company. Made to feelvery important and that he was making a contribution.
He was 'away' a lot leaving his new bride alone in a new country very different from Yorkshire, England, and with a new villa that was unfurnished and pretty empty, with black servants and no friends or neighbours she knew.
Meanwhile his African boss did everything he could to help. he took the new bride shopping for the villa, showing his generosity in his purchases for furniture & fittings, shopping expeditions in which he took her to lunch to places. He introduced her to some new white lady friends. White ladies already involved in the inter-racial circuit. He took her out in the evening to show her around the city so she would know where to take her husband, and introduced to his own wealthy African friends.
Within a month of her arrival she has been bedded by four Africans, including the house domestic the African had employed for her. The domestic had 'with the santion' of his boss spiked her food and bedded her while she was in a 'drowsy fog.'
Her husband knew nothing till the baby popped out ten months later, and promptly abandoned her in Zimbabwe flying home.
She decided taking a black baby home to her small Yorkshire Dale village abandoned by her husband after ten months of marriage was just not something to contemplate, while her new 'African friends' were supportive and helpful so she chose to stay.
I met her after all this had happened. She was 19yo, very pretty, healthy bouyant and energetic and in Zimbabwe without a work permit and now 'working' as a 'white *******' encouraged and protected by a major in the CIO.
I don't believe there were any underlying issues in their marriage, but of course for many there are..there are many reasons and those who have voted will have seen that the wide variety of reponses indicates that there is often not one issue that pushes at the limit. Indeed it might be a combination of issues that pushes a marriage apart.
Of course many will realise that interracial sex can often cement a marriage together and provide the glue that brings people back togther who might have been wondering how to satisfy their needs in a marriage where polite consideration of each others needs and worries about traditional attitudes had suppressed these gnawing needs..