It's just like any other fetishes. If it turns the couple on in pure clean fun, who's to judge? After all when u r 70 all u hv is memories of days gone by
Some interesting stuff bout cuckolding someone posted previously
http://www.cuckoldfantasies.com/about_cuckolds.html
For a more scientific insight
http://www.sekj.org/PDF/anz49-free/anz49-093i.pdf