Why.. ? Why..? Why..?

I couldn't sleep well ysterday nite after gone to my friend's annual Uni gathering at S's place.. Well this is what happen:
" I sms M (my ex uni guy friend who is now a married man) a few days back to give me a lift to S place.. if it was possible.. and He was ok to pick me up.. So he did came and pick me up.. Everything was cooled.. even till we reached S place and even when we left S place..
While he was sending me back home, he kind of touch my lap.. I looked at him and reminded him that you are a married man and a father of 1.. stop behaving like u still single and available..
M went saying that just trying to have a fling with you.. thats all.. I looked at him and said.. look I wont want to be any 3rd party of it OK!

M replied : " Have you ever asked urself what happen if u caught your another half having a fling or even caught them on bed.. will you forgive them?"
My replied: "Well I don't knw it all depends.. who did he slept with.. and etc... So how abt u will u forgive?"
M replied: "Yes, I would.. no one is perfect.. so thus I will forgive..
I went slient for a while.. so he is trying to tell me is not a problem for him after all.. and at then end of the day he will still go back to see e other half and said everyone do make mistake.. so please forgive me..

Well he tried to be funny with me.. I said please act like a married man.. and stop behaving like MBA ( Married but Available).. So eventually when I drop me at my house carpark.. I jst hurried and took my bag.. said bye to him even without looking or turning back to see him..

I guess a leopard will never change his spot even though he is a married man..

So yst nite.. I kept tossing and turning.. and wondering.. y ...? Y..? y cld someone do such a betrayer thing.. esp to their love one..