All religions had been created for some concrete reasons. Speaking about Islam, Muhammad made it in order to consolidate tribes from the Arabian peninsula around himself, and conquer the vast territories of his neighbours. So initially Islam was the religion, which had been created to unite...
It would be better not to give her this permission. Simply because I know myself very well. Even if firstly I allowed my spouse to have a sex with somebody else (because of some serious reasons, of course), then afterwards, anyway, I would feel myself betrayed, and I wouldn't trust her anymore...