Of course he presents himself as lovely, I'm sure he is, but behind closed doors he has been investing his time and emotional energy on you. That will definitely have had an effect on his marriage and if he is the type of person to lie and decieve his wife for 2.5 years he is the type to conceal how he treats his wife to others.
This, can you see this OP? Even if she thinks he's treating her well, the fact is that he isn't.
Every time he spends time with you, shares confidences with you, continues emotionally investing in you, kisses you, touches you, has sex with you - every time he does those things he is lying to her, betraying her and creating a moment that will devastate her if she finds out.
Her mental health will be taking a battering now if he's pulled away and become distant, or will take a battering if she finds out. You must have met her. How cruel to know what you've done and face her. Probably smiling - she probably thinks you're lovely.
You may well both be nice and lovely at other times but at the moment through your actions you are bringing out the worst in each other - a side in each other capable of lying and cheating. Everyone thinks their AP is different, the exception to the rule. But the rule exists for a reason.
You are naive to think you know this man as his wife does.
Also this. You know the bits of him he's chosen to show you. She has known him first thing in the morning to last thing in the evening in real, everyday life.
They have in jokes. Countless memories. A family. She knows his parents, siblings, oldest friends. They have kissed, snuggled, had sex - he has been besotted with her and said the things he says to you. And is either playing you by now pretending he only does / feels those things with you or he does only feel those things for you and is too cowardly to end it with her and too cruel to stop having an affair.