You may be thinking of Flesh, a 1960 novel by Philip José Farmer.
The space explorers return to a postapocalyptic Earth after a long journey:
Around and around the Earth sped the starship.
Where air ends and space begins, it skimmed from north pole to south pole and around and around.
Finally, Captain Peter Stagg turned away from the viewplate.
Earth has changed very much since we were here 800 years ago. How do you interpret what you've seen?
[. . . .]
"How do I interpret what I see?" said Calthorp."Your guess would be as good as mine. As Earth's oldest anthropologist, I should be able to make a fair analysis of the data presented—perhaps even explain how some of these things came to be. But I can't. I'm not even sure that is Washington. If it is, it's been rebuilt without much regard to the old city. I don't know; you don't, either. So why don't we go down and see?"
"We've little choice," sid Peter Stagg. "We're almost out of fuel."
Suyddenly he smacked his palm with his huge fist.
"Once we land, then what? I didn't see a single building anywhere on Earth that looked as if it might house a reactor. Or anything like the machines we knew. Where's the technology? It's back to the horse and buggy—except that they don't have any horses. The horse seems to be extinct, but they've got a substitute. Some sort of hornless deer."
They survived the long space journey by suspended animation, not relativistic time dilation:
"You're alive," said Calthorp. "And eight hundred and thirty-two years old, Earth-time."
"But only thirty-two years old in physiological time," Stagg said. "How are we to explain to those simple people that as our ship crept toward the stars, we slept, frozen like fish in ice? Do they know anything about the techniques of suspended animation? I doubt it. So how will they comprehend that we only stayed out of suspended animation long enough to search for Terrestrial-type planets? That we discovered ten such, one of which is wide open for colonization?"
A new religion worships Columbia, the Great White Mother:
Stagg turned red, and he growled, "I thought I was hardened, but those statues! Disgusting, obscene, absolutely pornographic! And decorating a place devoted to worship."
Calthorp shook his head. "Not at all. You have been to two of their services. They were conducted with great dignity and great beauty. The state religion is a fertility cult, and those figures are representations of various myths. They tell stories whose obvious moral is that man has once almost destroyed the earth because of his terrible pride. He and his science and arrogance upset the balance of Nature. But now that it is restored, it is up to man to retain his humility, to work hand in hand with Nature—whom they believe to be a living goddess, whose daughters mate with heroes. If you noticed, the goddesses and heroes depicted on the walls emphasized through their postures the importance of the worship of Nature and fertility."
Wikipedia plot summary:
In Flesh, Peter Stagg and a group of astronauts leave Earth in the twenty first century. Due to the benefits of hypersleep, they return to the planet eight hundred years later, in CE 2860. They find a strange world, inhabited by pagan cultists and bizarre societies decorating a scorched, rocky landscape, except for the mostly fertile eastern coast of the former United States and Karelians, European pirates from the remnants of Finland. Inducted into the mostly female "Elk" group, Stagg has antlers grafted onto his skull and is christened the "Sunhero". In that role, he becomes a sexual slave, forced to engage in intercourse with virtually every member of the group, especially virgins-although he balks at the "Pants-Elfs", a gay community from what was known as Pennsylvania in the twenty-second century. Society is dominated by goddess worshipping cults centred on the "Great White Mother" known as Columbia, as well as her adolescent daughter Virginia and the death-bringer crone goddess Alba. The primary thrust of the book's plot is Stagg's internal dialogue, ruminating on the moral, ethical, spiritual, and physical implications of his actions. However, when he truly falls in love for the first time, the object of his affections refuses to give in to his physical advances. Ultimately, Stagg, his newfound lover Mary Casey, the remaining members of his crew and chosen female partners and children escape from the neopagan-dominated primitivist Earth once more, en route to an inhabitable planet in Vega's planetary system. However, a denouement suggests that this may have been intended by the elderly priestesses who control this matriarchal society.
