I generally don't consider persons who fornicate to be immoral because society (much of it anyway) seems to have evolved in that direction. No one is really shocked any more that umarried people sleep together. It's become the norm.
That said, if one wants to be strict or technical about it and bring the Bible into it, then sex outside of marriage does fall short of the Biblical ideal. Then again, we fall short of that ideal in many ways.
I think a happy compromise would be to replace "marriage" with "committed relationship", which is pretty much what many religions have done.