Jamesrsl OK I am not going to be really hard on you and blame you a lot. I've no idea what has gone wrong in your marriage and I don't think it was wrong of you to have sex with your wife (although wonder if you have tried to address these issues before).
Let's think what your options are here. You could, if you chose to, think about how you could try and change this situation, for the better for you and your wife, and for your child.
Did you ever love your wife?
If no, then by all means check out now and be up front.
If yes, why does your post not mention any plans, or attempts, to save your marriage?
You've gone on together to create a new life, your son or daughter. Your first?
You are talking about all this as if you are changing energy supplier, and wanting to know what will be the best deal!
Please, please, do some soul searching, for your own sake. You did not have a sexual affair, a kiss is not the end of the world. Things are not irreparable between you and your wife, if you could get back the love you once had for her, and both work together.
Personally, I think you need to do some soul searching and decide if you really do want to end it and check out of your marriage.
If so, I would be honest with her. Be ready to answer her questions about what she has done 'wrong' (in your view), or what has changed.
Maybe she is fed up with you and will welcome the split. Or maybe she would be willing to work on things together.
Either way that baby is your baby, we assume, and you need to get your head around that and stop worrying about someone you worked with who made you feel good about yourself and kissed you.
Totally agree with nicenewdusters, "You say you still respect her. That's questionable. If you do, be honest and allow her the choice as to whether SHE stays with you or not."
and yes to LilyMcClellan "I love how this is described as a "terrible situation", like it was caused by some kind of unavoidable weather system passing through."
You need to take the responsibility that is yours, I don't think it is all your fault, I've got no idea what has happened in your marriage. But things have slid south and you have got involved in an emotional relationship with this other woman.
How do you know this other woman will want you. "Every so often we've spoken, but it's been over for several months. We stopped, because I felt terribly guilty, and I wanted to make my marriage work."
Why did you want to make the marriage work, because of guilt or a spark of love
Go figure out what is going on, be honest with your wife.
I hope whatever you decide you will speak about this in the most honest and respectful way possible.