Tobias Stanislas Haller's 2009 book published by Seabury Books, New York, is a book often referred to these days, I find. The reference is often of the 'Haller's arguments carry the day' kind, which means that not engaging with the arguments looks like one is avoiding the one argument or set of arguments which now controvert one's position.
So, here is a bit of engagement. But, watching some other time commitments, I will start slowly, and not argue back.
Chapter One "Where the division lies" notes, p. 3,
"So it appears that the argument from the conservative position is reducible to the irreducible fact of the sex of the couple - the sex difference must be present for a sexual relationship even to be capable of being moral, so that even if a same-sex couple possesses all of the other moral values, the lack of sex-difference still renders the relationship, and any sexual activity within it, immoral."
Haller then goes on to state two qualities inherent in the "sex-difference that is morally determinative in and of itself". These two qualities are, p. 4:
"that the purpose of sexuality is procreation, and only heterosexual sex is capable of it; that heterosexual sex represents a joining of two distinct complementaries."
This sets up Chapter Two which looks at the first of these qualities.