Most people are never going to believe that a romantically involved couple that shares a bed is not having sex. The trouble with this is that they add to the impression that it’s ok to cohabitate, and even have premarital relations. The fact that they add to an already corrupted cultural perception of sex and marriage is a grave enough matter, in my opinion, whether they are fooling around or not. If they’re actually going to get married, they’ll have the rest of their lives to share a bed. Why place themselves in a scandalous–and also very tempting–situation?
As to your quesiton of whether it’s mortal or venial, it’s true that a mortal sin places a person’s soul in greater danger if that person does not repent, but just because a sin is venial, doesn’t mean it’s ok to commit it. We should avoid all sins. The couple in question still shouldn’t be living together before marriage.