Steveno: Perhaps because of the lack of 'threat' the extra person poses. It's the 'safest' addition - no threat to the man's masculinity, no threat to his relationship, no threat to his heterosexuality. I know these aren't entirely believable or accurate (of course a person can ditch a heterosexual relationship for a same-sex one), but for many who would suggest something like this to a partner, it's fantasy initially, and nothing more, so the suggestion follows the fantasy. Not many people fantasise about things which might make them uncomfortable, hence the gender of the suggested 'extra'
My first post was meant as a joke, but in all seriousness, the reality is probably just that some heterosexual men fantasise about having two women in their bed, but don´t want to see their wife/girlfriend in the reverse situation.