“Sex Tape” is the kind of movie that gives sex comedies a bad name. I won’t dwell on its one-joke premise, in which a married couple make a sex tape and then realize that it’s spreading like wildfire to a multitude of iPads they’ve recently given to friends and family. Nor will I comment on whether or not I ever want to hear the words “Sex” and “tape” in the same sentence again. I like Jason Segel and Cameron Diaz and I hate to see them waste their time in a silly comedy like this, not to mention such talented costars as Rob Lowe and Jack Black. Notice I said “silly” and not “awful”. I’ve seen worse comedies this year, and so have you (hint: “Tammy”). But after watching this movie for a while I noticed I was marking time (and I rarely do that), just waiting for it to be over. A mainstream summer comedy with a solid cast should have a better story to tell shouldn’t it?