For your consideration, and I cannot provide links right here unfortunately, but call it hearsay or verify it yourself:
Surgeons found material in a female brain which had male DNA. If this is true, then seen consists of more than seed, hormones, and lubrication.
When a woman receives seed, she receives several other ingredients, which remain with her for life. She carries a part of that man, influencing her thoughts and feelings, for good or otherwise, for life.
The two have become one flesh in a real and irreversible way. Further, in principle, love flows down; respect flows up. We hear a lot about "mother love". No disappreciation intended, but perhaps a predisposition to love, nurture, and care for offspring is delivered with the seed. Sure, that's pretty woo-hoo.
I will try to investigate / substantiate / document. But it seems to me this answers to Paul's comments as well as anything I ever heard about union.