'She's just a friend' - why do some guys/boyfriends do this??

I have had a recurring experience with boyfriends in the past regarding their female 'friends'. The first time was with my first love eight years ago. A girl was very persistently pursuing him and when I questioned him about it, he denied there was anything between them - she was just a 'friend' he said. But I eventually got so fed up with her texting constantly/him doing nothing about it. This combined with other problems caused us to break up - and two weeks later he had jumped into bed with her! Eventually we got back together and left this girl, but it made me realise that he had lied about her being just a friend in the first place. The second time was with a bf of about 1 1/2 years. There came a point when lots of pictures were being uploaded of a particular girl lying on top of him etc, they were talking online and so on. When I asked him about it at the time, he said she was a friend. After we split up, she eventually became his next girlfriend. I recently broke up with my ex a month ago (relatively amicably). Before we broke up, there was a girl he had been talking to who was, you guessed it, 'just a friend'!! Well two weeks after we had been broken up from a 2 year relationship he is posting messages to social networking every day clear for me and everyone else to see. I felt this was hurtful and insensitive because it was so public for a start, and it made me think he'd moved on very fast. I eventually saw him in person and said I'd got rid of him off social networking for now because seeing that was too upsetting. He said 'she's just a friend' and 'there's nothing between us' but apologised for hurting me because it was so public. I didn't believe it, but accepted what he was saying. He even seemed to believe it himself. Guys, what's the deal?

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