three sum problem

So im in a bit of a predicament. Ive been married a year and half and have an 8month old daughter. Recently my husband and i have been mulling over the idea of having a FFM three sum just to spice things up a little. The IDEA wasnt bad to me, but i also never thought any of the people we knew would be up to our standards, so i never thought we would find anyone. My husband (without my knowledge) put this up on two different apps geared towards this, and actually had a girl write him back, who was interested. She is very pretty. He called and told me he thought he found someone while at work but didn't elaborate. When he got home, he showed me the apps and then showed me the convo between them. It included normal conversation which turned to naughty conversation and saying baby back and forth and THEN they started sending inappropriate pics back and forth. I was ok with finding someone, but feel so hurt that he would do all this without my knowledge. I tried to not tell him i was upset because i maybe this is how these sort of things start. But all night until i went to bed (and AFTER) they flirted and sent pics and vids. She then asked if my husband would do anything just them two, which he answered maybe. The next morning i saw he wrote her that i was asleep and he wished she were here to do things with her. Im so hurt. But i also told him i wanted a threesum. So is this how this starts? Am I wrong for feeling upset? Is he wrong for doing this? Is this cheating? I cant help but feel uninvolved and liked hes cheating. Please help. I still want to do this, but hate the way hes going about it 😭