You are only letting me choose one. There are numerous reason why I looked for an affair. The thing that stands out in my mind was that I wanted to find out if there really was such a thing as good sex.

My marriage consisted of sex 3-6 times a year for 3 minutes from start to finish. I thought that was normal.  Now that my eyes are open I will never go back.

I was looking for sex, companionship, someone to do things with. Just to touch another human.

Life is too short not to enjoy all that it has to offer.

I'm with Lily a bit - there were many reasons, and really the list is a bit short for me.  I was on a slow burn for a long time (15 years!).  But now that I'm 50+, I just got to a point where I couldn't just rot off the vine, and had to do something for myself.  
Even when we were newlyweds, my wife never had much of a sex drive; we were on the sort of plan Dick was talking about, except less often, and I could tell she was faking it.  
Then, about 4 years into the marriage, my wife had to have a hysterectomy, and after this , her sex drive went into park!  I felt very sorry for her for a long time, and due to her not being a very good healer, what sex we would have after the operation and recovery was painful to her (that was her report).  
So, I backed off and went into a hermit's existence for the next 10 years!  Tempus Fugit.

Then, at about the 10 years married mark, and through another source (my wife's old college roomate) I found out that my wife knew about the condition that led to the hysterectomy, and that the condition ran in her family (fibroids)!  It probably could have been handled earlier (like before we were married), and if that had happened, there would have been a better chance of a pregnancy.  As it was, the condition never came up in our going out/engagement!  That bit of news from her old roomate was a major revelation to me, as before we were married, this information indicated that my wife knew that I wanted kids, but she didn't let me know about the probability that would not be possible, due to her condition!  So, it was pretty clear that she knew she probably wouldn't be able to have any kids, and she just let me think we could have kids together, which was a major reason I got married to her!  So, I was reeled in a bit and got married into a prison that would not be easy to live in.    (Cont'd in next post)

(Cont'd from above)
We have talked a bit about the nature of our relationship, and I am very lucky, actually, to find someone who had a similar sort of situation to myself, and who was also willing to take a chance and be vulnerable - and start looking to take care of their own situation!  We believe that it was no accident that we were brought together, and will stay at it, stay discreet, and see how deep the rabbit hole goes!  We know we have a lot of lost time to catch up on, sex-wise.  I talked to her a bit about this 'shadow marriage' idea, and I think she likes the concept.  We'll have to work on it, and develop our own style.
Also, thank you, Dick, for this site, as, until I found it and some of the discussion here, I wasn't quite so sure that I should get into this relationship; but I'm glad I have, and that there is a way forward besides the prison I was letting myself live in.