@itsstrangebutitstrue
You are very young. A lot is going to change over the course of your relationship but that does not mean that either you or your husband will have an affair. The key is to keep communication open and if you see/feel there's is an issue (especially after children) then immediate open a dialogue about it and seek outside help (couples therapist for example)
Living in fear is only going to adversely affect your relationship. Constantly worrying where the other person is or what they're doing etc. Enjoy yourselves, take, explore each others's minds and bodies, travel, relax, do all the things that strengthens your bond.
To answer your original question, yes affairs are extremely common. Much more so than people realise. I work in a profession where this is information I would be privy to. People don't tell their friends/family/confidants because they are embarrassed. I know of many couples who are referred to, in the town where I am from, as "ideal", "so amazing", "perfect for each other" A lot of it is show. They are, behind closed doors, struggling.
People who believe that affairs are uncommon are lucky enough to not have had one touch their lives directly or have not had it shared by friends/family. It is one thing, I have found, people are most reluctant to talk about. And you can quote this if you like and tell me your friends/family/circle are beyond reproach and squeaky clean but I would almost put money on it that there's someone everyone knows who has been unfaithful. It happens for so so many reasons, although on MUMSNET there is only one reason, that the person is a hateful person. This thinking is rife with judgement and bitterness and damages the person themselves. Anger is the punishment we give ourselves when someone else makes a mistake.
OP try not to worry about it. If some day something like this knocks on your door you will deal with it, none of us know what's waiting around the corner tomorrow so be mindful, enjoy the days and weeks and months. Life will happen one way or another.