So what is your objection to using biology as a basis for morals? Especially as you have no objective criteria to offer.
There is a difference between what is necessary for biological life and what is the supreme good. Biology dictates that we must eat to live but does eating make us good? What if one eats too much to the point of gluttony? In your position, you say, biology dictates that Animals reproduce through sex but does that make them good? Are two dogs mating in the front yard morally good for following their biology? No. Good is more than biology. If it were mere biology then God would be doing it himself.
Seneca argues that the supreme good comes from soul processes which humans are uniquely created to do. We apply reason, wisdom, or moral consciousness to biology, to nature in order to produce good in others and in ourself.
“This being so, you should consider whether one has a right to call anything good in which God is outdone by man. Let us limit the Supreme Good to the soul; it loses its meaning if it is taken from the best part of us and applied to the worst, that is, if it is transferred to the senses; for the senses are more active in dumb beasts.
The sum total of our happiness must not be placed in the flesh; the true goods are those which reason bestows, substantial and eternal; they cannot fall away, neither can they grow less or be diminished. 17. Other things are goods according to opinion, and though they are called by the same name as the true goods, the essence of goodness is not in them. Let us therefore call them "advantages," and, to use our technical term, "preferred" things. Let us, however, recognize that they are our chattels, not parts of ourselves; and let us have them in our possession, but take heed to remember that they are outside ourselves [in our flesh]. (Seneca, moral letter #74)