Lab partner again .Does anyone get it?

He is 38 years old I was 42 when this happened.I think if your involved with someone you owe it to them to tell them things you do.Especially when its another woman.If it ever surfaces like It did .It looks bad.As far as trusting him.When i meet him he had a girlfriend who just cheated on him.HE felt bad.Well then at work we hooked up.He felt bad and told her the next day she came aftre me I told him stay away.And he did for about six months and then we hooked up again.HE did nt tell her for about 4months .So this makes me uneasy.He said he knew it was comimng to a end with her since she slept with another man.Besides she was like 12 years his senior and very unattractive.So I do think about this .So it can make me feel there is no trust also.I never expected him to hide this from me or not share with me we were always close.And then you get some peopel on here saying she liked him.And some people say she was just a trying o be a friend .Mayeb if you peopel knew him.He is such a avoider and a introvert you would be surpreised liek I was.So why cant anyone answer teh question insted of passing judgment.