In the moment, when you are both getting off on it, everything was fine. The problems seem to have develop for you after the fact. Your conflicted feelings on what took place are completely understandable. However, unless you work through them they do have the potential to interfere with you enjoying the experience again. That would clearly be a shame for both of you
So, what to do?
First, I think you need to talk to your wife. Explain to her how you feel about what happened, both how turned on your were in the moment and how you felt after. Be open and honest, but not accusatory in anyway. This needs to be a conversation that helps both of you understand what each other is experiencing and feeling and with that in mind, encourage her to explain to you what she loved about it, how it made her feel, and what she particularly enjoyed. The more you are able to understand how it works for her, the easier it should become for you to fulfill her desires and your own.
Second, you need to try to think about it differently. Your wife, I suspect, does not feel insulted by what took place; she does not feel like you have abused her or harmed her. In fact, the opposite is probably true. She feels hot, sexy, and turned on. You need to focus on that and on the fact that you enjoyed it, too. Exploring your sexual turn ons together is a wonderful, powerful thing to do.
In that moment, your partner is not a mother. She is a sexual being who wants to be treated as such. When you are having sex I am fairly sure that her being a mother to your children is the last thing on her mind. She wants to be a sexy, naughty, dirty woman and she wants you to be turned on by that. I am fairly sure if she did not love and trust you to share this with her in a way that makes her feel good, then she would not have suggested it. Just like if she asked you to give her a massage or play with her nipples, she is expressing her needs and desires to you: her sexual partner. I think it is a wonderful thing that she was able to do that. Many women and men find being open with their partners about their desires very challenging and often chose not to share things for fear of being rejected or judged negatively.
