NC for this. 6 months ago I had a gut feeling that something wasn't right with DH. Being secretive and protective of his phone which he never has been before. I checked his online account and there were hundreds of texts to and from one particular number over a 6 week period. I checked his phone and there was only one text from this number, a female work colleague. The text mentioned them "overstepping the line."
I immediately confronted him and he said that it was a friendship that had got out of control, he was sorry, he'd been stupid but he was flattered by her attentions etc. he would tell her that it had to stop and he'd been a twat, he didn't want to lose me and DD.
I have checked his bills and there has been no further communication other than the odd sporadic text/brief call which could be work related, who knows?
He went out on Friday night with some work colleagues. When I asked who was there he reeled off a list of names and tagged her name on the end. I wasn't happy but he says as they work together she's going to be at those functions and there's nothing he can do about it - he says he barely said 2 words to her. But instinct told me to check his phone again this morning. She sent a text very late last night which was unopened, so I read it:-
"I have been thinking about this all day. I think it's only fair that I am totally straight with you, so you know exactly where you stand. I can't allow myself to be friends with you and I will continue speaking to you in work as little as possible. This is as a result of you getting involved with me when you had no intention of ever leaving your wife. You hurt me and I feel upset, as all I was and would be, is a fuck on the side. I admit I still really care for you but I deserved better and I won't be used again."
I threw the phone at him and watched the colour drain from his face as he read it. He says that she is infatuated with him, they had a massive argument when he told her they couldn't be any more than friends and that she is bombarding him with texts which he ignores and deletes. He maintains nothing physical happened, but she asked him to leave me for her on several occasions (she must have been bowled over by his charm and witty banter) 
He's gone to work in tears saying that he only loves me and that she is trying to wreck his marriage because he's told that he doesn't want anything to do with her.
We've been together for 14 years and have a 3yo DD. I have resigned myself to the fact that he did have an affair which I think I can handle, but only if I can get him to admit that it actually happened which he won't do.