‘My Partner Wants A Threesome, Should I Fulfil His Fantasy?’
Ideally, early on in a relationship (and perhaps before you get engaged), Purnell says couples should check that they’re aligned on the non-comprisable issues, such as level of commitment (monogamous/non-monogamous), family expectations (children or no children) and their expectations/desires for sex.
Article source: https://www.huffingtonpost.co.uk/entry/partner-wants-a-threesome-but-i-dont_uk_6256c3c1e4b0e97a35193c28