It means a poor forecast for a successful marriage. The second way into trouble takes two. If they use each other for sensual gratification, for premarital sex, then it will feel like love. There will be a feelings of warmth, tenderness, desire for closeness and the like. But it will be only chemistry.
Real love is unlikely to develop in such a setting.
For they are putting each other into such a state that if death came, that one would be wretched for ever. That is not desiring the happiness of another. It is closer to the opposite. As we said, it is using another. Besides the offense to our Father, there is another tragic side to such a picture.