He was caught off guard

I knew my man has been cheating on me. I saw him one time busy chatting with someone in an online dating site where I also met him. Our first affair happened in a foreign bride social where we got acquainted and fell in love. The rest is history. I was in the room when I caught him off guard, sweetly chatting to a woman he met in that site. I was really angry and pissed knowing we are married and expecting our second child. The nerve! What should I do? His explanations are not even satisfying at all! Please help!