Instead of defending him so much, tell us more about how you feel. This is new to you but clearly not to him. He should have brought up the desire before he fell in love with someone else. Also, a lot of people here will refer to it as NRE (new relationship energy) over being in love.
Talk to him. Tell him right now you aren't comfortable with this and you need time to talk and understand.
"Regret for the things we did can be tempered by time; it is the regret for the things we did not do that is inconsolable." - Sydney Smith
Kyle: 27 year old male
Katie (rymmare): 25 year old female
Kids: girl: 5 years old, boy: 3 years old