An interesting point of departure. Please don't go.
men and women are genetically different. XX vs XY chromosome pairs. *** and XXY and YXX occur, and are called trisomies, and are known to be correlated to specific differences from the regular XX and XY.
men and women are developmentally different, largely as a result of effects of hormones on specific tissues, including the brain.
One of the largest concerns we should have as a society is the effects of chemicals we create as they become our environment. Plastics of some types are "plasticized" with phytoestrogenic compounds like bisPhenol A, and these compounds are being detected in soil and water and have been observed in some areas being associated with feminizing certain species. The real danger is we are losing our real men, and what all they are worth to families and society. Here we've got a whole lot of men who are just off the track hormonally and can't, or perhaps don't respond normally to natural stimulii that promote the actions to, function as men. I don't think that's a plus, personally. I don't think it's "equal to" being a functional man, so to speak. Definitely not what I want for my life.
Biologically, actions that are not somehow helpful to reproduction, are not sexual actions. Well, in the least they are not the effectively "right" actions.
I know some women who, I suspect, would like to marry their cat, and who in fact do put animals in their wills. Some men, in Wyoming according to some accounts, have true love for sheep. I don't think their wishes are any less valid than anyone else's, and I don't care to involve myself in their private lives, but I'll be damned if I have to get all timid about my "bigotry" for thinking they're not as acceptable or beneficial to society as a man and woman who are living for one another and for their children, working to be beneficial members of society. There is no reason not to honor them as good examples, and no reason to try to make anything else a "marriage".
Precisely because it is a distinction that has a purpose. It's not even a "purpose" invented by man, it's just something coming from our natures, from nature itself. All we're doing with structuring laws that deal with it as a special case is recognizing the fact of nature, and adapting our laws to that situation, with some purpose in mind. Like showing kids that moms and dads are important, and helping moms and dads take better care of their kids.