You think the movie can't be that bad with such a cast. You've got Robert De Niro, Diane Keaton, Robin Williams (R.I.P.), and Susan Sarandon, all VERY successful actors, plus Topher Grace and Amanda Seyfried, who are not as established but are quite well-known (I don't add Katherine Heigl to this list because I can't believe she's actually famous. I have never liked any of her movies, she can't act, and she's not even that good looking!). But the talents of these stars cannot save a poor script or a bad plot line.
It's an awful idea for a movie, so it's just ridiculous all around. First of all, it is grotesquely sexual. From the get-go, you've got De Niro going down on Sarandon in the kitchen. Um, what? Later on Grace is getting a hand job under the table at the reception dinner (real classy), and although there are no real "sex scenes," the EXTREMELY loud orgasm sounds are so over-the-top (somebody's clearly faking it...). It's just distasteful.
Also, this movie is SO predictable (aren't all rom-coms these days?). As soon as you see Heigl not feeling well in the hospital nursery and then throwing up on her dad, OF COURSE you know she's pregnant. I mean, come on. If you didn't get that immediately, you are a moron. And it was so obvious that Mr. Grace (the virgin doctor? Yeah...right.) was gonna get it on with the slutty, exotic Latina. There was no mystery here.
And the relationships between the characters are so weird. Keaton and De Niro are divorced because he cheated on her with her best friend (Sarandon). But then Keaton and De Niro have sex. Sarandon is upset but forgives them, and Keaton is just so happy that her ex-husband is marrying her still best friend even after the cheating. Oh, and Keaton admits that she had an affair with her son's fiance's dad. WHAT?! You might as well have thrown in Williams as the Catholic priest into that mix if you're going to overdo it like that.
So pretty much this stupid, vomitous movie is telling you that adultery is okay because at the end of the day everyone is happy. You could throw every cliche ever into a romantic comedy, and you would get this movie. Except instead of feeling happy at the end, you feel like you can't get those 89 minutes back of your life.