Probably. Most men keep secrets from their wives - whether that be online, offline, or in another dimension.
It is normal to want some privacy - and let me tell you now, every man makes mistakes and indiscretions. But you can only tell whether these indiscretions are serious in nature through other signals that he might be subconsciously omitting.
For example, if he had setup an account on a dating website to meet other women, then in your home life he many seem more distant and distracted, less affectionate and so forth..
They only way to know for sure is to keep tabs on his browser history in that case.