Marriage is NOT and has never been a "love certificate". It serves a real function.
Historically, that group has always been heterosexual pairings (and in some cases heterosexual groups). It is not simply a forgone conclusion that the interest of the state in granting marriage privileges to heterosexual couples automatically extends to homosexual couples -- there are important differences between the nature of these pairings, regardless of anyone's opinion on the morality of homosexual activity. The question, then, is "is it in the interest of the community to promote homosexual pairing by providing special privileges to homosexual couples?"
In my opinion, it's not in the community's interest to do so. There are many reasons. One is that homosexuality is considered immoral by large swaths of the population, and that official sanction and promotion of homosexual pairing will anger this significant segment. Most of this segment is, however, at least less opposed to civil unions and the granting of certain default privileges that decent persons believe ought to be granted to persons who live as a couple, regardless of their affiliation or activity.
If the most vital privileges can be granted with a "lesser" legal status, and we can strike a reasonable compromise that promotes peace and civil discourse, isn't that better than choosing the more divisive, incendiary method?
I believe if the debate really were about basic privileges related to emergency medical notification, etc., gay rights activists would wait to push the "marriage" terminology. I think that the debate is actually more about validation; homosexuals demand the government grant them and their relationships equal standing and promotion as it grants heterosexual relationships. Gay rights activists are thereby attempting to use the government as a weapon to cram their concept of the equality of homosexual relationships down the throats of an otherwise skeptical public; over and over again referenda have shown that majorities in most states DO NOT want this to happen.
There are many other implications that flow from state equivocation of hetero and homo pairings. Religious groups may be censured for preaching doctrine that the state has deemed illegal; they may also be censured for exercising that doctrine, i.e. excluding practicing homosexuals from the clergy on grounds of unworthiness. In Massachusetts, we've already seen the complete withdrawal of Catholic Charities, as the courts attempted to force them to adopt to gay couples. This trend will become broader as gay marriages are legalized, recognized, and normalized, and may culminate in the denial of many religious services to the residents of many states.
Whether you think the religious are right or wrong in their assessment of the morality of homosexuality, you should be willing to protect their privilege to exercise and proclaim their beliefs that do not directly result in the termination of life or the loss of right and control of property, just as you expect the same privilege for yourself.
It's worth noting here that the attachment of negative attributes to homosexual practice need not be strictly associated with religion. An independent observer may likewise rule homosexuality irregular, abnormal, or otherwise not something worthy of active state promotion via marriage. Homosexuals cannot bear natural children. Homosexual couplings cannot provide the duality of permanent male-female parental role models, which I believe most of the West now wrongly considers worthless since so many were raised in single-parent homes. You may think this trivial now, but the fact is a huge component of the organization and legal recognition of marriage was the effect of parental partnership on children. Does the community really have an interest in equalizing homosexual and heterosexual couples, when homosexual marriage cannot produce any children in the first place by its very nature, and when any adopted children are automatically disadvantaged without even the prospect of opposite-sex parental role models (a "mommy" and a "daddy")?
I could go on; as I said, there are many angles to take on this, and many sides from which it appears negative. I understand that some people are eager to use the government to enforce their particular vision of equality on everyone else, and probably mean well, but that is not how a free society operates; you must convince people on your own merits, you don't just go and get the government to endorse your beliefs and then attempt to intimidate otherwise unwilling participants with that endorsement.