The right to marry has been historically and remains the right to choose a spouse and, with mutual consent, join together and form a household. FF 19-20, 34-35. Race and gender restrictions shaped marriage during eras of race and gender inequality, but such restrictions were never part of the historical core of the institution of marriage. FF 33. Today, gender is not relevant to the state in determining spouses’ obligations to each other and to their dependents. Relative gender composition aside, same-sex couples are situated identically to opposite-sex couples in terms of their ability to perform the rights and obligations of marriage under California law. FF 48. Gender no longer forms an essential part of marriage; marriage under law is a union of equals.What I'm going to talk about below may delve briefly into some descriptions that may involve sexual terminology; parents of young readers are cautioned in case they don't want their young ones seeing some of these words as of yet (though I won't be unduly or gratuitously graphic).
An opposite-gender couple, when they enter into a marriage contract, are entering into a state in which they promise certain rights to each other. A central right, in fact, the one that makes marriage different from most other types of legal relationships, is the right to engage together in the act known as coitus or sexual intercourse (which used to be referred to delicately as "the marriage act" in a less promiscuous age). This right is granted mutually (e.g., husband to wife and wife to husband) and exclusively (at least as far as the law is concerned--that is, there is no such thing, as yet, as a legal right to marry more than one person at a time).
So important is this right to marriage that most states consider non-consummation, under at least some circumstances, to be grounds not for divorce, but for annulment. A secular annulment (like a Church one) is a statement by the competent legal authority that no marriage ever took place between the couple. The use of non-consummation of marriage as grounds for annulment varies by state, but many states posit that the inability to engage physically in the act of coitus, if known by one partner and hidden from the other prior to the attempted marriage, may indeed be grounds for such an annulment.
What does this mean? It means, in essence, that heterosexual couples are exchanging the right to sexual intercourse with each other, and that failing to disclose that one is incapable of this act at the very least, or the incapacity to engage in this act at all whether disclosed or not (in some states) renders the marriage invalid--null and void. Does this force every married couple to engage in sexual intercourse? No--but if they do not, if one of them cannot, and if the other takes his or her case to court, the state may very well rule that they were never married at all.
How does this relate to same-sex "marriage?"
Clearly, a same-sex couple cannot, by definition, engage in the act known as coitus or sexual intercourse. Thus far, this objection is dismissed out of hand by most pro-gay "marriage" types; gays can have sex, they say, so there's no issue here. But what same-sex couples mean by "sex" is not coitus or sexual intercourse (obviously). They are referring to other sex acts, specifically anal sex (men) and mutual masturbatory acts performed on each other in various ways (both male-male and female-female couples). They cannot be said to be "consummating" their "marriages" at all.
Why does this matter? Because as the law stands, a heterosexual couple can have their marriage declared invalid, null and void, for the non-completion of an act that same-sex couples are by definition unable to engage in with each other! This is not equal treatment under the law--this is placing a specific burden and obligation on heterosexual couples that by definition cannot be placed on same-sex couples.
I know that the same-sex side will argue for a remedy: simply do away with consummation as a factor when considering the legal status of a marriage, or else define "consummation" to mean the engagement by the couple in any sex act, including anal sex etc. The problem with either of these proposals is that they both do a specific harm to opposite-sex couples, who have always been able to annul a marriage in the event of certain factors centered around non-consummation, defined as the non-engagement in the act of sexual intercourse following the marriage.
If an opposite-sex couple marries, and, let's say, the wife has hidden from her husband a condition which makes her permanently incapable of engaging in sexual intercourse, and the husband seeks legal remedy regarding the marriage, it makes a big difference to him whether the law recognizes his marriage as invalid, or whether he must divorce his wife (and possibly be liable for alimony etc.). The law's recognition of the central importance of the act of sexual intercourse to marriage, and its upholding historically and up to the present day of the right of a man or woman to have his or her marriage annulled under some circumstances surrounding non-consummation, supports this notion. It is a positive, if putative, harm to opposite-sex couples if, in a post-gay "marriage" world, the ability to seek annulment for non-consummation is permanently removed from them.
But if opposite-sex couples retain the right to annul marriages for non-consummation according to the laws of their states, and concurrently face the peril of having their spouses seek this legal remedy for non-consummation, but same-sex couples are neither permitted this right nor face this peril, then opposite-sex marriage and same-sex marriage remain fundamentally and unjustly different according to the law.
And, as I mentioned briefly above, the right to annul a marriage for non-consummation may be restricted or limited by the states in some ways, but the right has not generally been abrogated altogether; moreover, it is a right which is historically present in regard to marriage, certainly since the earliest days of this nation, and present in some form or other dating back hundreds of years--so stripping it from opposite-sex couples is doing them a very definite harm, by changing in a dramatic and irreparable way the rights and obligations of marriage in a way that puts them in a specific jeopardy.
I recognize that same-sex "marriage" advocates are going to object; I expect that some will do so because, after all, marriages aren't annulled every day of the week for non-consummation. That may be true, but it's not important to the central point: opposite-sex marriage and same-sex "marriage" are fundamentally different concepts, which at their core are centered around a fundamentally different obligation as regards the sexual rights and duties of the married couple toward each other. In the simplest possible way to put it, opposite-sex couples are, essentially, at risk of annulment if the couple does not engage in post-marital sexual intercourse or coitus with each other; there is no identical obligation or identical risk for same-sex couples, and there never can be.