In this episode, Melody speaks with a couple of people in heterosexual relationships whose attractions are nevertheless far from straightforward.

"Alex" is straight but sometimes sleeps with men, Linda Jane and Eddie look like a straight couple but one of them identifies as queer, plus sex therapist Nic Beets and his daughter Lena helps us make sense of it all.

If you're about my age - that's 33 - and you had parents who acknowledged the existence of sexualities other than straight, you probably grew up thinking there were three sexual orientations - homosexual, heterosexual and bisexual.

But more and more we're hearing sexuality described as "fluid". What does that mean? Can a person identify as 70% straight, or mostly gay? Can you start off life straight and then identify as gay or bi at different points throughout?

Or is the whole notion that sexuality can fit inside one tidy box or another completely outdated?

Clinical psychologist and sex therapist Nic Beets thinks it is, and welcomes the move away from allocating our sexualities to discreet boxes.

"It's really easy to judge other people when you don't know their experience so I would always caution against that... People are doing things sexually for all sorts of reasons and in all sorts of ways," he says.

People like Alex (not his real name), a Generation Xer and kiwi 'bloke' who identifies as straight, but whose personal history is far more complicated. He got in touch with me to share his 30-year experience of sleeping with other men.

Alex is only ever attracted to, and is only interested in establishing emotional relationships with women, but every now and then he gets the "urge" to have no strings attached, "base-level, animalistic sex" with other men.

And he does, by using a network of websites and known public places that put him "within a vicinity of other men with similar interests", and which he says many other men in straight relationships use too.

"There'll be people that you work with that'll just slip away at a lunch time, literally go and have sex, be back after lunch and the people around them will be none the wiser." he says.

Nic Beets says the phenomenon of ostensibly straight men sleeping with other men is well known by sexuality experts and academics, and that the practice is, "as old as humanity".

"In my understanding, straight men are doing this for sexual gratification because it's straightforward and easy and they don't have to pay for it. It's very much about "getting off"... other men will give you the kind of sex you want." …

Go to this episode on rnz.co.nz for more details