Ok, the truth is out. And my suspicions WERE correct.
A few months ago I notice on my facebook new feed (yes, it involves facebook) that DP had become 'friends' with a girl's name I didn't recognise. Didn't particularly think anything of it. Over the next few days it kept showing up in my news feed over and over - 'DP is now friends with X', making it seem as though they kept ceasing to be friends then becoming friends again. I asked who she was, and he said she had sent him a friend request so he'd accepted, had a look at her profile, decided he didn't know her (her location is not out area and they have no mutual friends) so deleted her, but that she kept sending friends requests and they somehow kept becoming friends and he kept deleting her.
So last night, I was on the laptop, he was watching TV (same room). His facebook was logged in but I was looking at something else. A friend request popped up from the same girl. The name triggered my memory and I thought 'who IS this!?' so decided to accept the friend request so I could have a quick nosey at her profile then delete her. As I clicked 'accept' I said to DP 'that girl has added you again, I'm going to check her profile out'.
He suddenly jumped up and started saying 'no, no Millington, don't, she just keep bothering me, saying all this stupid stuff'. As soon as I had clicked accept, a message came through from her on facebook chat - 'oh wow, thanks for accepting, I sorry if your girlfriend's there but I wanted to speak to you, have missed you'. Straight away I was like 'erm, what's she on about?'. DP continued to act squirmy, saying 'she keeps sending me messages like this and bothering me, I have no idea who she is'.
I sent a few message back, with DP standing there next to me, such as - 'I don't know you, please stop adding me' (her reply - 'is someone with you right now, you're acting weird, don't you remember me?'), and 'how do we know each other?' (her reply - 'we've skyped a few times, you liked my pics and showed me your dick'). With that last one I replied 'please stop adding me' (her reply - 'no no, wait I'm sorry!') then clicked both delete and block.
DP continued with his story about her being some stranger who was bothering him - even saying it might not even be a girl, but someone out for a laugh winding people up. But something about his reaction seemed a bit off. I asked him if he even had a skype account, as I have never heard him mention it or known of him using it. He said no, he didn't have skype. Then a couple of minutes later, he 'remembered' - oh yes, he has got a skype account as he once had to use it at work a while ago but has never used it since. Everything seemed to be pointing one way and that's when I posted my OP about trust on here.
Anyway, this morning he was still maintaining the story about this girl being some sort of stalker. He was talking about how much he loved me and how he could understand that I why I would be upset about this but there was nothing to worry about. I asked him if I could have a look at his skype account. He had 'forgotten' his password so had to get it emailed. I asked him to log in to his email to retrieve the skype password, but while his email was logged in I searched for this girl's name in his inbox.
There it was. A message from facebook saying that she'd sent him a message - the email showed part of their facebook conversation.
Him: I like your pics, they make me really hard.
Her: I could see that, you make me really horny.
Him: Did you see my cock?
Her: Yes it was huge, Can we go on webcam again?
Him: My girlfriend is back soon.
Her: Ok, our little secret ok?
I am absolutely devastated. I showed him (all he could say was 'I'm so sorry') then left for work straight away, where I've been ever since. He's been texting - I'm so sorry, I just did it once and felt so guilty afterwards, I just want to move on, I love you so much' etc.
We've been together for 5 years, lived together for 3, bought a house together last year. No DC. Sex much less regular than when we were first together. As well as 'chatting' to this girl, it's the lies that have really got me as he denied it all night and probably still would be if I hadn't seen that message.
:(