Men is it difficult to remain faithful and content?

Someone I have been friends with and meet up with once every 6 months or so has just propositioned me.
He is engaged, just had a 3 month old, brought a house together 6 months ago...

From my understanding they were happy.

We only meet up to do art type things together and catch up.

Now he's flirting and propositioning me...

I'm not bragging. I'm really hurt and incredibly upset.

I thought he was one of the good ones.
4 years they've been together and never once have I tried to over step the mark.
He's advised me and listened to my relationship issues like a big brother!


I have told him off now we will had time amo