I'll comment on the four rules for sex by atheist philosopher Keith Parsons:
Hey, all God's children got rules. I think that most nonbelievers follow the same sort of rule with sex that Budweiser recommends with beer: Enjoy responsibly.
Here are four rules I think that most unbelievers would endorse:
1) Sex should be between consenting adults.
2) Use effective contraception.
3) Practice safe sex.
4) Respect marital commitments.
That pretty much sums it up. Simple, effective, and moral.
By contrast, over its history, here are just some of the sex practices the Christian Church has forbidden or at least disapproved, even when practiced in private by consenting adults:
vaginal sex for any purpose other than procreation.
sex between males
sex between females
sex between unmarried persons, even adults ("fornication").
even "impure" thoughts
Wow. Is this the "tradition" you defend?
Let's go back through the list:
1. Why would most unbelievers endorse #1?
i) Is Parsons appealing to moral intuition? If so, historically, many cultures have sex slaves. That's not consensual. Presumably, cultures that have sex slaves don't think it's morally wrong to have sex slaves. For instance, it's traditional for armies to take women as booty. The "spoils of war". If Parsons thinks that's wrong, he needs to explain why so many other cultures don't share his moral intuitions in that regard.
ii) Peter Singer defends bestiality:
Is bestiality consensual?
iii) Does Parsons think consent is a sufficient condition for morally licit actions? Take teenage boys who dare each other to perform dangerous stunts. That's consensual. But does that make it morally licit for me to encourage someone to perform a stunt that might leave him dead, disabled, or brain-damaged?
(If you think teenagers are too young to consent, I could easily substitute college students.)
Suppose I offer a recovering junkie very pure cocaine or heroine (or the drug of choice). He finds the offer irresistible. Was it wrong of me to get him hooked all over again? After all, his acceptance was consensual.
Suppose an older brother (17) is a drug user. His younger brother (15) wants to know what it feels like to trip out. The older brother lets the younger brother experiment with his drugs. Is that morally licit? It's consensual drug use.
iv) What about college students who go to parties with the intention of drinking to lower sexual inhibitions, and hooking up. If the strategy succeeds, is that still consensual? If not, why is that wrong from a secular perspective?
2. #2 is odd. Why is contraception a rule for having sex? Is Parsons an antinatalist?
3. He says practice safe sex, yet he defends oral sex, anal sex, and sex between men. But those are examples of risky sex. Consider the many health hazards.
4. It's unclear what he means by respecting marital commitments.
i) Is he alluding to adultery? But what if consenting adults decide to have an open marriage. Marriage gives them reliable companionship. A reliable sexual partner. Someone to fall back on. But if this life is all there is, why pass up opportunities for extramarital sex? Just do what comes naturally in the heat of the moment. You won't get a second chance.
ii) Furthermore, adultery is between consenting adults. So how does the rule #1 relate to rule #4?
iii) Or what if a couple commits to monogamy, but after a few years of marriage, and the passion wears off, one or both spouses want supplement their staid sex life with extramarital exploits.
iv) Or what about polygamy?