So, outside of religion, is there any basis for a personal restriction on premarital sex? Are there any objective limitations for sexual behavior?
I would argue that, outside of the prohibitions of religion, there actually are still objective reasons to moderate one's sexual behavior. Of course there are the obvious ones, such as risk from STDs and unwanted pregnancy, but I think there are also emotional ones. Although emotional/psychological impacts of certain behavior can't always be quantified, I think we can at least make empirical estimates of what they might be.
I don't think it's always an issue of certain behavior being "right" or "wrong". Rather, I think it's kindof like the Buddhist concept of karma - whatever choices you make, there is a "karma" associated with it. So if you have sex with random chicks you meet at the club, that creates one type of karma. If you only have sex with one person in a mutually monogamist relationship, that creates another type of karma. It's just up to you to decide which type of karma you want in your life.
And although we can't always clearly delineate the tangible consequences of different behaviors, I think the differences do exist. It's like choosing to go left or right on a path - either direction will take you somewhere, it just depends on where you want to go.
As far as objective limitations for sexual behavior, I think that objectively we can say first and foremost that anything that harms someone should be considered wrong (but that's probably obvious).
Also, sex with kids should be considered wrong because even if they are "mature" and willing, they are not as mentally developed as an adult, no matter how mature they are. As long as we don't live in primitive societies where people die in their thirties, kids should be able to grow up without the influence and pressure of someone else's sexual desire imposed on them.
Sex with animals should be considered wrong because they can't give consent (Again, I'm just trying to lay out a rational standard for behavior here without invoking religion).
Incest should be considered wrong because of the conflict it creates with the structure of family relationships. Intimate relationships can come and go, but families are usually involved and support one another throughout one's entire life. Plus, you don't want any children with damaged genes.
I think those are the big issues that religious people argue would be allowable outside of religion. But there seems to be plenty of reason to not do them regardless.
Also, I think the current standard for becoming sexually active should be whenever you can support yourself financially. I often hear people talk about becoming sexually active when someone "feels" they are ready, or when they meet someone "special". Until you have a job and ARE paying the rent, I kindof think that's bull****. If you're dependent on your parents (or anyone else), or you don't have money to pay for a kid if pregnancy does happen, or to pay for treatment for an STD, then you are not "ready" no matter how old you are or how "special" the person you're with might be. I think if more people had that standard then that would solve a lot of problems in society.
Alright, that's all I got. Thoughts?