The arguments for heterosexuality being based in biology usually run something like the following:
- Male and female reproductive organs are naturally compatible with each other, rather than with themselves.
- Male and female intercourse is the only natural way of producing offspring.
- As offspring are the only way for the species to perpetuate its genes, it's logical that those genes would predispose their donors toward heterosexual behavior.
Meanwhile, the arguments for homosexuality having a biological basis goes something like this:
Arguing that one must choose to be straight or gay (or bi) is good for a philosophical discussion of the issues, but it's pretty hollow when it comes down to a serious discussion of biology and/or genetics. One of the few things both creationists and Darwinists agree on is that heterosexual behavior is the "default setting" for every species that uses sexual reproduction, because otherwise the species would have long since been replaced by the amoeba.