I struggled with this too. I'm 2 months from dday (discovery day). Getting men (I am generalizing here so men please don't get all pissy on me) to open up and express feelings that relate to failure, grief, and weakness is challenging on a normal day, almost impossible after dday. So, don't judge him for not just pouring out his story and feelings to you right away. It'll come in small slow pieces and you may never know everything you feel like you deserve to know.
I'm going to give a shot at this, but I'm no therapist so I'll just tell you some things that worked for me.
1. Ask why he gave himself permission to do this. (I got that from a website) He'll probably say that he doesn't know. Just ask him to think about it and let him know that you won't judge him for whatever his reason is, you just want to support him so you can build a stronger relationship. A couple days later ask him again. No matter what his answer is, it is going to seem trivial and not a valid reason - because really there is no reason that is good enough. This is your opportunity, however, to build his trust in you to express his feelings. He is probably very confused right now, grieving over lost love of the OW, wondering why he did it, etc.
2. Ask him if he used protection. Every time. Is he sure? (Then still go to the doctor and get yourself checked out, just in case.)
Think about what you really need to know. As women we often want to know all of the details. There are many details I asked about that I really don't need floating around in my head right now and I would have been better for not have asked.
3. I asked if he loved her, but you already know that he did so you can skip that one.
4. Ask him if he is maintaining NC (no contact).
5. Ask him if on your wedding day he ever thought he would have an affair. ( I haven't asked this one or the next one yet - use at your own risk

)
6. Ask him what changed. (If you went 7 years since your A, it may not be that. The lack of communication or some other issue may be the cause and that you had one made him feel like he could have one.) It may be that you need to have some counseling so you can talk about both A's and truly recover from both.
7. I asked when did it start and when did it end. It helped me to know that when I suspected it started and validated my feelings that "something was going on".
My problem is that I'm not always ready to hear the truth. So, I've told him not to tell me anything that I don't ask for unless there is something that he needs to get off of his chest. Now I'm asking small detail questions to practice my response so that I can demonstrate to him that I can respond without crying, yelling, screaming, or using the information he gives me to make him feel like a jerk later on.
A couple of days ago I asked him where he put his wedding ring when he went to see her? In his pocket, in the safe. I felt that tinge of anger that he treated our symbol so flippantly. Took a deep breath set the anger aside and asked if he had told her he was married why did he take the ring off. He said it was just too weird to wear it. So he knew right from wrong, he took off his ring and transported himself to fantasy work and then just like the power rangers put it back on and zapped himself back into the boring existence of our marriage/reality (we'd only been married 7 months, together for 3 years - not sure how it got boring that fast). And, I wonder if she really knew he was married. But, I didn't say any of that, I just put it away and said I understood and then went back to talking about other things. Now I must be vigilant in not using this information when I get sad or angry.
I hope some of this helps but if it doesn't seem appropriate to your situation, just toss it!
