Wait, not that kind of co-ed shower! We mean one to which the groom is invited.
No boys allowed may still be the standing rule for bridal showers, but what is this, 1890? If the bride is marrying him, we can assume that she at least likes the guy (who, by the way, is not getting a groom-al shower of his own).
In the spirit of inclusiveness, a co-ed couples shower is an opportunity to give gifts from the registry that will benefit the bride and groom individually, but would be weird to present as a gift to the couple (read: nose-hair trimmer). Co-ed showers can also balance out the party with a few well-behaved boys if assorted in-laws and hangers-on are threatening to make it look like a snoozer.
Plus -- and this should not be overlooked -- the bride gets more presents.