"Proponents pointed only to a difference between same sex couples (who are incapable through sexual intercourse of producing offspring biologically related to both parties) and opposite sex couples (some of whom are capable of producing such offspring.) Proponents did not, however, advance any reason why the government may use sexual orientation as a proxy for fertility or why the government may need to take into account fertility when legislating." pg 122
In other words, you made precisely the same fallacious argument that the proponents of Prop 8 made in this case. That fertility needs to be taken under consideration with same sex couples even though fertility is not taken into consideration with opposite sex couples. No opposite sex couples are denied marriage on the basis that they are infertile, and so there is no basis to deny same sex couples the right to marry on the basis that they are infertile. That is not equal protection under the law. That is discrimination based solely sex and sexual orientation.