Well i'm not for it either but you could i suppose use the following:
Chemical reactions, that's a physical chemical thing. A thermonuclear/biochemical reaction. Or thermodynamic equation. Endorphins. An axon synapse interaction. Hormonal.
http://www.txtwriter.com/Onscience/Articles/Lo veChemistry.html
Runaway sexual selection driven by a positive-feedback mechanism.
The evolutionary propensity towards heterozygousity.
Link it in with survival of the fittest, primal instinct and evolution, the requirement to procreate to prolong the species, a biological need and genetically programmed.
You could expand on monogamy here for pureness of breeding and such. A process designed to eliminate undesirable genetic mutations.
A sieve to ensure highly pathogen-resistant offspring.
A psychological illusion. A fantasy by those who are insecure and hate solitude, need attachment, a form of social, religious and cultural acceptance and conditioning.
Not my views by any means, just trying to help :) x