Considering ending it with my finance . Am I over reacting ?

Im very upset . Im sorry in advance for my typos .

4 years ago, my university classmate ( we were doing our master’s) , Tyler, whom I had a huge crush on asked me if I wanna go to his work party with him. We went there and had a great time. We didn’t even kiss so I knew it wasn’t a date. Then later he texted me and said thank you then asked if I like to hang out on the weekend too. We went bowling then for a walk . We realized we have a lot in common . Again he didn’t even kiss me and I assumed he either sees me as a friend or he was very respectful. Then kept inviting me to do stuff together almost every weekend . I ended up asking him if we are together and exclusive ?. He danced around it and changed the subject then texted me 2 days later and said he wanted to be exclusive with me and liked me.

Fast forward 4 years, we have been engaged for 1.5 years and I’m pregnant ( due May 12th). We were invited to a dinner party ( mostly his friends from his old job) . One of the guests accidentally revealed that he was never interested in me initially ! He initially asked me for his work party because his gf at the time canceled on him ! All those hang out stuff was while he was cheating on his gf! Then only when I asked him about us being exclusive , he ended with his gf the same night

When we came home I told Tyler that I’m disgusted by him. He lied to me. He said he didn’t lie because I never asked . Also, no cheating was involved because nothing physical happened between us back then.

Am I over reacting ? Am I crazy to think he was not an honest guy ? He thinks it was long time ago and im being hormonal and over sensitive over nothing