Thank you all so much for your responses. I had a really good think and came to the conclusion that I really was open to a poly relationship. I felt quite excited by the idea that it could bring a new openess and dimension to the relationship and when he admitted he had been seeing someone although annoyed at his dishonestly I found I was not jealous but keen to get to know this other person.
But then he came out with more things he had lied to me about and at first he said he was keen on a primary secondary structure and my ideals on having kids suited him then back tracked on both these things. I decided enough was enough and we should call it off before we caused each other more pain.
I really believe honesty is the most important aspect of any type of relationship and am very sad that I discovered he was incapable of it. How could a poly relationship work when one person is so full of lies?! I did truly discuss everything with him and gave him every opportunity to be honest. I have a feeling that in truth I was not as important to him as he made out. Mrfarfromright you hit the nail on the head, he met a person in a poly relationship whilst travelling and seemed just concerned with that. I have to say I am still open to a poly relationship, just one established on truth and honesty.