Your argument is based on what you consider natural.
The next step in your argument is that homosexuality is unnatural. That they are outliers. You dehumanize and then devalue their relationships.
The next leap in your argument is to associate every other type of relationship that you feel is unnatural.
However you don't seem to be able to separate out issues.
All of the relationships that you mention lack consent; incest, 70 yo marrying a 10 yo, child brides & child pornography lack consent. The law protects against vulnerable people being exploited.
The slippery slope argument is pathetic. It is pathetic as it avoids the only question up for debate which is "Should SSM be allowed to marry" yes or no.
