I have a good friend who I've known for a long time. I love her and consider her a very close friend, but she's also someone who seems all super open on the surface but there's all kinds of stuff going on below.
I have long had my suspicions about her DD. Since she was really very small, I kept being struck by how much she looks like a friend of ours. My friend and her DH are very close to this man, and have been for as long as I've known them. Recently, the man posted a picture of HIS DD who is about 5 years older than my friend's DD and I almost dropped my phone in shock. I showed DH and when looking at the picture, after a few seconds he said, "is that friend's DD from a few years ago?"
I think it's entirely possible she had an affair. I think it's possible her DH knows and has forgiven her EXCEPT that he is still very close with this man which seems odd. So my new theory is that for whatever reason, they asked him to donate sperm.
Obviously, if she wanted me to know, she'd have told me and I'm certainly not going to ask her. Ditto, I would never ever say anything to any mutual friends or even just anyone in real life in case they ever met her/her DH. So I came on here! 
Any other ideas on how this might have happened? Affair (known or not) and sperm donation are my main ones. I guess it's possible they used to do threesomes back in the day too (the other man was single until a few years ago but is very attractive).