Sex is certainly present in Cat's Cradle and not always in its classiest form. Adultery and prostitution both show up. But Vonnegut isn't porn-mongering here. Those aspects of human sexuality exist in the story because Vonnegut is interested in human existence, and human existence isn't always pretty.
Here's an example of what we're talking about:
I will not go into the sordid sex episode that followed. Suffice it to say that I was both repulsive and repulsed. (118.17)
The sexuality is present, but John doesn't linger lustfully on the act or provide tawdry details. Instead, he presents it in a way that allows discussion without distracting from it. We're not quite sold on the PG-13 rating, but it's definitely not R-material.