Not having a intimate relationship could or could not be a sign. Everyone is different. I guess I would have to ask you how long you have been together and your ages and if it just stared..
It's a funny thing about affairs. After the affair is found out a lot of little things start to surface that was going on with him/her and they make sence..
hidding atctivity on a computer and cell phone are one. losing weight and working out. Trying to look good when they go outside the house, more than they use to..
It some cases more sex, And in some less.

PM me if you want to go into details.. I've been there..