Men's sex habits explained

I once dated a guy who wasas obsessed with having a threesome. He made a list of candidates, scouted locations, ran ideas by me - he had the event planned from beginning to end like a wedding.

I was more perplexed by his behaviour than put off; I can see the appeal of a threesome, but why the fixation?

I realise that my ex is not alone, every girl and her sister has had some guy hint that he'd like to hop into bed with her ... and her sister.