From the web site about.com on the matter of how to word wedding invitations:
Optional details include telling your guests what to wear. Strictest etiquette tells you not to include information about attire on your invitation, but I think this is an outdated opinion. Guests appreciate clues about how to dress, and are not as instinctively knowledgeable as they used to be. To avoid someone showing up in blue jeans, include a line such as:
Other options: Semi-formal, cocktail attire, festive attire, creative black tie, white tie, black tie optional, dressy casual, informal.
The same web site provides the following definition of "semi-formal":
Semi-Formal or After Five means that tuxes are not required, nor are long dresses. An evening wedding (after 6 PM) would still dictate dark suits for him, and a cocktail dress for her. Daytime semi-formal events mean a suit for him and an appropriate short dress or dressy suit for her.
And while I am sure you will say that you do not consider this to be your definitive guide in such matters, that isn't the point I'm making. I'm saying that one cannot assume that everyone is reading from the same book. All I know is that I've been to more than a few "semi-formal" weddings where a suit was what was being requested.
Heck, in many of these "semi-formal" weddings the groom was wearing a suit and I don't think I would want to show up at someone's wedding in a tuxedo with the wedding party in suits. Like I suggested previously, the issue isn't about who is technically right or wrong.