The World Federation One Stop Fiqh

Ruling 2447

If in order to become maḥram a father or a paternal grandfather marries his non-bāligh daughter/granddaughter or son/ grandson to someone for a short period of time, the marriage contract is valid as long as it is not detrimental. However, if during the period of the marriage the boy is totally unable to derive sexual pleasure, or, if no sexual pleasure can be derived from the girl, then the validity of the marriage contract is problematic [i.e. based on obligatory precaution, it is not valid].