Daisy,
your partner was brave in telling you, and I think you should see that as a sign that he wants to talk and put things right, you say he wants things to go back to normal, can you try find out what is going on?
Speaking from the perspective of someone who has snogged somebody else, from the standpoint that this was not just a drunken snog (because that is a different and not so serious matter), realizing you have feelings for somebody else than your partner is not necessarily fun, it can be heartbraking, and love is not logical as Divastrop said. He snogged her, he did not have sex with her. He told you, and he wants things to go back to normal, I take that as he wants things to go back to BEFORE he snogged her.
Sure he can be blamed for allowing himself to fancy her (if he did), and for snogging her, but it is up to you and him what you do about it.
In my case, my marriage came out a lot stronger as I had to put considerable time and effort into figuring out WHY it happened, what was going wrong in my life for somebody else to be able to enter my thoughts, and thereafter time and effort in talking to my partner about what we could do to make our relationship and our lives better. So, yes, I did cheat (if snogging is cheating, and I think in my case it can be classified as that as it was NOT just a drunken kiss) but the experience made me realize what I value in my marriage, what sort of marriage I want to have, and how to better take time with my partner and communicate with him, and finally put things behind me. I dont recommend snogging as a way of improving your marriage, but how you deal with the aftermath can improve your relationship.
You cant just sit there alone and feel sorry for yourself and wonder why, you need to get him to talk. At least you need to get him to THINK and ponder these issues. And you may need some councilling to put you and him and your relationhip back on track.