Though not much romance is to be found in this pair the relationship would last long enough. Aries would be too romantic for the Capricorn man to handle. He is usually reserved when it comes expressing one's sensuous feelings and emotions. But he tries to show his love for her by raining material benefits on her and not by his emotions.
An Aries woman and a Capricorn man do not make good friends for life but they make use of the each other's company wisely. If each is honest then the relationship can thrive on a kind of friendly note.
This is a good combination for marriage. But it has to do more with a traditional, conventional sort of marriage. Marriage usually has to deal with raising children, improving the finances of the household and maintaining the status of the family in the society.
Sex comes naturally for this combination. It would be a predictable act for both involved in the relationship. The Aries girl would also look out for something new and the Cap if can feed her senses, it would be well and good for the duo.
This would be a stable and strong relationship as Capricorns usually cling to what they hold forever. Only the Aries woman if she feels getting bored has to call the last shot.