CULTURE: Same-sex marriage? Euthanasia? Child's play issues in the avant-garde philosophy of Peter Singer, the "most influential" philosopher alive. Parental warning: This article refers to infanticide and some abnormal sexual activities. Don't expect Peter Singer to be quoted heavily on the issue that roiled the Nov. 2 election, same-sex marriage. That for him is intellectual child's play, already logically decided, and it's time to move on to polyamory. While politicians debate the definition of marriage between two people, Mr. Singer argues that any kind of "fully consensual" sexual behavior involving two people or 200 is ethically fine. For example, when...