Suppose my preference for mobility is genetically informed. Does that mean "know your customer" laws are a violation of my civil rights? If sexual preferences turn out to be malleable by circumstances and are not genetic, would that make gay marriage restrictions acceptable? If not, then I don't see the relevance of genetic influences on human preference.

If you believe there is no fundamental principle preventing the state from regulating same sex marriage and the state merely needs a rational basis to do so, here is a very simple one. It incentivizes bisexuals to lean straight, thereby reducing the amount of gay sex and the transmission of STDs.

Note that I don't subscribe to the reasoning I just stated - that's just standard left wing political theory (which I don't agree with) applied to a scenario that left wing types refuse to apply it to.