I think it's pretty serious too. I'd have major reservations about marrying him. Some women have said that it's okay becuase it's fantasy...well, unfaithfulness starts somewhere and I bet it rarely starts as a full-blown affair. Also, consider your own feelings. If it were me, I would feel as hurt as if it HAD been a full blown affiar. I've read of women on here (upon discovering their husband is looking at porn) feeling horrible, being upset, sad, and worthless and feeling like they've been cheated on (but are confused, casue it wasn't a "real" person)...It hurts... It really hurts because it IS real. Your feelings ARE real, not "fantasy."
For my husband and I, faithfulness begins with the heart and mind. Porn is an addiction, and it that ever crossed our path, we would recieve marriage and addiction counseling because it would affect both of us in different, disturbing ways. We had that convo before marriage, and thankfully he is a man of character and realizes that it is an addiction and to stay away from it like the plague. It ruins marriages and self-respect.
I know this may not be the "politically correct" or "popular" opinion...but really? Really??? It's acceptable?? I personally believe men are capable of harnessing their minds and bodies. I support my husband in being and becoming a wonderful man, father, and husband who will teach our daughters to love and respect themselves and teach our sons to cherish and respect women...and that starts with how the father treats his wife. "Real" or not, porn is not respectful towards women nor being fully faithful to wives. Also, to those to see no harm- addictions tend to escalate. When a certain level of promisuity doesn't "do it for them" anymore, they seek harder stuff... and then harder stuff... and sure, some may never cross the "virtual" line, but by then, they've eroded their own self-character so far that they aren't the same person. This stuff is SO destructive to the self and to relationships... why would anyone want to take that chance? Just google porn addiction and family... to see the terrible stories and also ways to help your loved one.
That being said, we all have faults. Give him a chance if you feel that he is enlightened or that this was just an experiement. Give him all the tools to understand how to fight against it and understand just how harmful it is for a family and your relationship.