Rick Warren defends his opposition to same-sex marriage using very standard arguments. He likens it to other "disapproved" relations, including incest, polygamy and pedophilia. This is a classic maneuver by opponents to gay rights. If gays and lesbians receive the right to marry (or engage in consensual sex, adopt kids, etc), then this will inevitably lead to legal acceptance of incest, pedophilia, polygamy, bestiality, and a host of other horribles. Although slippery slope arguments are seductively rational, they fail in two crucial respects.
Source: Sorry, Adam and Steve: If You Get Married, We Must Allow the Smith Triplets to Wed Each Other As Well!
Slippery slope arguments fail because they assume that no material distinction can separate the "new" right (here, same-sex marriage) from the list of items that exist on a perilous slope (pedophilia, etc). But even if Warren and others lack the brain power to see the differences between me walking down the aisle with a male partner rather than my mom, pet dog, or 3 siblings -- I can happily make distinctions among these choices. Many relationships on the slope, for example, involve coercion; accordingly, we prohibit pedophilia because children cannot adequately consent to sexual relations with adults. The slippery slope argument only works if we close down our minds to the possibility of material distinctions between same-sex marriage and the parade of horribles.
Slippery slope arguments fail for a second reason: they do not demonstrate why the preferred right (here, heterosexual marriage) does not place us on a dangerous slippery slope. But most incest, pedophilia, and polygamy involve heterosexual males exploting younger women. Accordingly, if any marital relationship puts on a dangerous course towards incest or child rape, it heterosexual, not gay, marriage. Of course, I do not link these relations because I, unlike the opponents to same-sex marriage, have not blinded myself to the possiblity of logical distinctions.
Source: Sorry, Adam and Steve: If You Get Married, We Must Allow the Smith Triplets to Wed Each Other As Well!