Among the more foolish slams against traditionalists is that our views are arbitrary religious dictates disconnected from realms of clear reality like science. Folks who believe that trope would likely find Faye Flam’s mention of homosexuality in her recent op-ed on male behavior to count as evidence:

I also learned there’s abundant homosexual behavior in male animals. Killer whales and manatees engage in gay trysts, while gay geese and ducks latch onto one another in devoted male-male partnerships. About 8 percent of domestic rams are gay — a persistent source of frustration for sheep breeders.
There are many theories about the persistence of homosexuality in nature — but one of the most interesting connects it to the power of diversity, which gives creatures the flexibility to adapt to different circumstances.

If homosexuality is natural, the errant thinking goes, then it ought to be fully accepted, and such relationships ought to be treated in like kind to the closest heterosexual relationship. That is, marriage should be redefined as an intimate pairing so as to incorporate the natural affections of gays. As it happens, I happen to agree that homosexuality ought to be accepted as natural, although I don’t believe the government should strive to force any more than civil tolerance among those who do not accept it. On a personal level, I’d encourage homosexuals toward the strictures of what I believe to be an accurate religion, but in similar fashion to my encouragement of heterosexuals whose behavior is immoral by that measure.
On the marriage count, though, I’d raise a subsequent paragraph from Flam:

Other men just want to have fun. One man I interviewed admitted to having sex with more than 200 women by the time he turned 40. But he was ready to change — and hoped to find someone to inspire him to settle down. Others may start out devoted to one partner but then their circumstances change — they get elected governor of some state — and they start mating with other women, too.

Society’s project is to mold rough human nature toward healthier, more productive ends — to learn over the millennia what practices are fruitful and which are detrimental. Marriage is a mechanism for just such a molding, so the fact that an impulse or desire is natural has no bearing on whether marriage ought to bend in its favor. Marriage is meant to pull that philanderer into the devoted relationship into which he says he’d settle down if somebody “inspired” him so that children aren’t left without fathers and mothers without support for their children.
The plain biological reality is that these concerns do not exist in homosexual relationships. Other concerns do, and ought not be sloughed away, but insistence on total equivalence would be a reckless response to the existence of partial similarity.