1) Think of the children!!
Which ones? The ones who you keep telling their orientation is dirty and shameful and sinful and who see it will result in a lifetime of fighting for basic human rights, the ones who are bullied at massively higher rates than everyone else, who have much higher suicide rates, who are sexually assaulted at higher rates, who are kicked out and left homeless at higher rates?
No? Oh, then you must mean the ones whose parents have to keep trying to cobble together a series of documents to attempt to legally protect their family and their custody, the ones whose families may be sharply redefined simply by moving one state over, the ones whose parents are rendered invisible or who are held up as examples of what we can't let the world turn to, whose parents are compared to child molesters? Oh, except it turns out there's actually no evidence they have poorer outcomes. They're not more likely to be gay themselves (if you consider that a negative outcome), and they're not less likely to be well-adjusted. They're just kids, and legitimizing their families would seem like a great conservative project if "family values" hadn't acquired a bizarre definition.
Oh, so it sounds like you mean your children, who might—horror of horrors—be forced—forced, I say!—to read a book where someone has two mothers or where two penguins raise a chick together. And then they might—oh no, anything but that!—ask you a question or something. So by "think of the children!" you mean "I don't like having to talk to mine."
Dictionaries don't create meaning, they report it (sometimes poorly). Not that "marriage" actually has meant much of anything we currently use it to mean throughout human history, since, after all, its a concept humans invented to serve their own purposes at varied places and times. The Bible alone reports polygyny, concubinage, and forced marrying of deceased siblings' spouses, none of which we're anxious to include today. Sometimes spouses have been selected by parents* and sometimes they were self-selected. Sometimes marriage was a practical economic arrangement (division of labor). Sometimes people paid to marry someone, and sometimes they were paid. Sometimes marriage is assumed to be monogamous, sometimes not. (Sometimes it actually is, sometimes not.) Sometimes it's meant to be permanent, sometimes not. Sometimes the participants have to be of a certain age. Sometimes they have to be of the same race or caste. Sometimes they have to be of different sexes. Sometimes one member controls another, sometimes they're seen as equals. Sometimes they have to make religious vows, sometimes only secular ones. Sometimes children are required or at least intended, sometimes that's irrelevant. Sometimes control of property and inheritance was the primary goal, sometimes sex was, sometimes love was. Sometimes you share a bed, sometimes you don't. Sometimes you like each other and talk over breakfast, sometimes you go months without seeing each other. Sometimes you can't have been married previously but sometimes that doesn't matter. Sometimes you can't be married currently; sometimes that too doesn't matter. Sometimes you have to sign things, sometimes you have to break things, sometimes you have to light things, sometimes you jump over a broom. Sometimes there has to be a clergy member or pillar of the community to pronounce your marriage valid, but sometimes it's only between you and the other party. Sometimes it's a partnership, sometimes it's a hierarchy. Sometimes you're in love (sometimes before, sometimes just after). Sometimes it's a community issue (even/especially the consummation), sometimes it's private. Sometimes it's a legal process, sometimes it's a religious process, sometimes it's a personal process; sometimes it's some combination. Read some books on the history and functions of marriage. Or just go look a married gay couple in the face. (Other countries are already doing this. Some states in this country are too. Married gay people exist, so obviously the term "marriage" already means them. You have already lost this battle.)
To be continued, naturally.
*I was going to put links for every single thing, but that gets old. Everyone's capable of Googling, I hope.