The slippery slope argument

[sd_bear]

The slippery slope argument is baseless.

Can a dog/goat/cow consent to marriage with an adult? No.

Can a child consent to marriage with an adult? No.

Can two male adults consent to marriage? Yes.

Can two female adults consent to marriage? Yes.

Can one female and one male adult consent to marriage? Yes.

What’s the argument here? How does this open up anything that isn’t opened up already?