I've been on MN for 8 years. I've seen so many posts from people who have been cheated on and the common line is as my title. No. Nobody thinks their partner will / would cheat on them. Especially when you have just had a baby / been together for 20+ years / been childhood sweethearts.
But every week I see threads where women are cheated on with work colleagues / via dating sites / etc. There is not one person in the land who can truly say their partner categorically would not cheat on them. You only know yourself. You do not know how anyone else thinks. No matter how much you might 'know' them.