Self-gratification is the natural end of same-sex acts. That is, same-sex acts are ordered toward self-gratification.
Procreation is the natural end of opposite-sex acts. That is, opposite-sex acts are ordered toward procreation.
One can only make the case for parity between opposite-sex acts and same-sex acts if one denies that procreation is the natural end of opposite-sex acts -- that is, if one denies the natural order. IMO, such a denial is completely unreasonable.