Each time Mac and Kelly have sex on-screen, something goes hilariously wrong. The opening scene has them trying to have impromptu sex on the table, except that Stella is happily watching them, even when turned around. The second has them going at it pretty well but ignorant of the frat watching from outside the window until they're well into coitus. The third is when both are sloppily drunk and try to have rough sex in the kitchen; when Mac asks Kelly to hit him, she grabs a nearby spatula and starts painfully whacking him with it.
Stella eating a condom.
The scene at the hospital proceeding it.
Doctor: Your baby, has HIV. (Kelly gasps)...is how bad this could've gone. The condom was unused.
Kelly: WHO THE FUCK SAYS THAT TO A COUPLE OF PARENTS?!!
Pete is dismayed at Mac and Kelly's temporary breakup, commenting to Teddy that, as a child of divorce, he sympathizes. Teddy says, in the most empathetic voice imaginable, "Well... my parents love each other. And I think this is hilarious."
During the fight between Mac and Teddy, Mac wields a small trampoline as a shield when Teddy throws an unopened can of beer at him. It flies right back and nails him in the head.
When the buyers see Stella playing with Kelly's dildo:
Mr. Baiers: We have one like that at home, except it's black.