Ok
First, DF and I DTD one night and the condom broke. I went into the chemist the next moring for morning after pill (and got grilled, yay) and anyway, took it within about 8 hours of it happening and I ended up Pg. So, the morning after pill isn't always 100% effective.
Personally, and I think I'm going to be in the minority here, I would keep the baby. Regardless of who's it is, it shouldn't have to suffer for a mistake it didn't make.
If you are that against telling your husband, then don't, and this other man should be cut out of your life if this is a mistake that you only intend on making once. At the end of the day people only tell their partners of their cheating to ease their own guilt and nothing good ever comes of it and it ends up hurting the people you truely love.
I really do feel sorry for you, it would be very stressful thing to deal and I think you are truely regretful but it is your mistake and you should have to wear it.
That is my two bobs worth, I'm sorry if I sound blunt but I just thought I would put another angel out there. And remember whatever choice you make will be the right one for you, so don't agonise about your decision after you have made it.
Good luck with everything! xx