Most people also agree that some people are suited to be parents, and some aren't.  We don't really do anything formal is our society to prohibit people from having children, or punish people who have children when they shouldn't (except when they cross lines such as physical child abuse).  That's a political choice.  We do generally say, "A married father and mother are generally better for children", so we provide legal and social benefits for married people.  But we don't say, "If you get married you must have children" and we don't even say, "The willingness or ability to have children is a requirement for marriage", so the relationship between marriage and children is somewhat tenuous.  We also don't say, "Drunks, people with abusive personalities, swindlers, and crack-heads can't get married or have children", but we do say, "Gay couples can't get married".  All of the quoted statements in this paragraph are political decisions, largely based upon the collective moral code of the people who pass laws, but still, they are reached through discussion and consensus.  Almost all of them are arguable, and, like the choice of 18 as a voting age, or 16 or 18 as driving ages in different jurisdictions, most of them are subject to change.

Look, even murder statutes, which are based on a fundamental premise of "thou shalt not kill", vary from place to place, and, in every jurisdiction that I've heard of, some kinds of killing (self defense, non-negligent accidents, etc.) are tolerated.  (Not to mention war and other police actions.) "Thou shalt not kill" is NOT an absolute moral principal!  It is a guideline that almost all of us take very seriously, and that we interpret politically.

Marriage and sex and child-rearing are the same way.  A hundred years ago, it was legal for first cousins to marry in many jurisdictions, and, today, it is illegal in most.  Interracial marriage is still illegal in some jurisdictions, and the age of consent also varies from place to place.  Many people equate spanking with child abuse (and not nearly all of those people reject strict discipline; some just don't think that spanking is a proper and moral tool to use to enforce discipline).  Curiously, Ohio is now debating a law that would decriminalize the abandonment of newborn children by their mothers.  Evidently, too many girls have successfully hidden their pregnancies from their families (usually their parents, but in some cases, their husbands) and, fearing being found out if they go through the legal hassle of adoption, they have simply abandoned their babies.  But since abandonment is illegal, whether it is in a dumpster or at an adoption agency, church, or hospital, some of them have chosen to dump their babies in secret to try to avoid detection by either their families or the police.  It's a horrible and immoral situation (typically, but not always, preceded by out-of-wedlock sex).  But still, as the number of dead abandoned babies accumulates, the political winds have swung from "mothers must care for their children or turn them over to someone else through a deliberate process" to "we realize that some mothers are physically or emotionally unable (or otherwise unwilling) to deal responsibly with their babies, and we've decided that we need to allow them to turn them over to someone else anonymously ".  Some legislators think that this is a terrible, irresponsible, immoral law, but, ultimately, the decision about where the law will come down is a political one.

Look, I know that when rules change, people get confused.  It happened when interracial marriage became more widely accepted, it happened when "The Pill" was introduced, and it happened when prohibition came and went.  In some ways our society is more moral than it used to be, and in some ways it isn't.  What I can say is that - to me - a marriage between two loving, dedicated, committed men or women is a lot more moral than a marriage for profit, an adulterous marriage, or an abusive marriage.

Maybe homosexuality cannot produce children, but then again, a lot of heterosexual couples can't either.  So where's the sin?  I know gay married people who suppressed their nature and got married and had children and are now tormented by the pain that they have caused (or will cause) their spouses and children as they finally lose (or expect to lose) the strength to carry on that "unnatural" relationship.  But still, those people can be good parents.  Gay couples (as well as gay single people, for that matter) have raised their own children or adopted children and have done a terrific job.  (Typically, of course, gay people are only allowed to adopt otherwise unwanted children.)  And, of course, I know or know of plenty of "faithful" heterosexual married couples who have done a horrible job of parenting.  Strong-breadwinning-man-nurturing-stay-at-home-woman is a fine model, but that model doesn't fit everyone who has children, nor does it work every time it does exist.  So whatever the "ideal" might be, gay does not "equal" wrong!  Conscientious people strive to be the best that they, as individuals (or couples), can be, and few of use reach the "ideal".  (I'm flashing on a movie scene in which a character protests "But I have a wife, two kids, and a dog", as he's being hauled off to jail for some offense.  I can't remember the details, but the key is that the "ideal" is good, but - for individuals - it's neither necessary nor sufficient!)

I'm not saying that you can't look at two specific people (gay or straight) and say, "Those people are not sufficiently moral, or mature, or intelligent, or whatever, to have children.  I just saying that you can't wrap a string around all gay people and say, "It is wrong for them to marry and/or to have children".

There are inappropriate heterosexual feelings, and there are inappropriate homosexual feelings.  But if sex were just for procreation (within a marriage or not, for that matter), then, of course, infertile people whether of "childbearing age" or beyond menopause would have no reason to behave sexually.  So procreation, and thus gender roles, are not essential for marriage to work! 

Finally, I'm going to put forward my basic argument for the acceptance of civil same-sex marriage.  Adulterers, thieves, the greedy, drunkards, slanderers, swindlers, all can get married whether repentant or not, and whether they have any intention to honor the standard marriage vows.  Bank robbers, murderers, tax cheaters, child molesters, people who have no intention to be faithful to their spouse, murderers on death row, bigots, jerks, cowards, and liars can all get married, too.  But gay people can't.  Not even those whose motivation is exactly the same as many heterosexual couples - to consecrate a stable, supportive, loving relationship that will last "until death do us part".  And then, the fact that they are unmarried is turned against them when "sexual morality" and "the breakdown of family" is brought up.  It's not fair, and it's counter-productive.