Because if homosexual behavior is genetically-determined (or inalterably biologically determined) the gays can argue that they are being discriminated for what they ARE, rather than for what they CHOSE to be. There's a large number of laws which prohibit voluntary behavior of one sort or another even if there are no direct victims (drug use would be one example) so the proponents of restricting homosexuality would have it easier to argue that there's an overriding societal concern which justifies this voluntary behavior. (The existence of laws creating "victimless crimes" is an accepted part of modern legal systems, sadly, but no one other than hard-core libertarians actually oppose the concept.)