- If it's "anything goes!" so long as you aren't already closely related, legal recognition of gay marriage will lead to all sorts of terrible things... bestiality marriage, polygamy, pedophilic marriage, exploitation by straights for monetary or other gains, etc.!
Rest assured, all of the age, species and other restrictions that currently apply to opposite-gendered marriages, would also apply to same-gendered marriages and all of the ways people might exploit same-gendered marriage are already being exploited by people in opposite-gendered marriages.
All of the things that those who oppose same-gendered marriages are afraid same-gendered marriages might lead to can already be arrived at, by exactly the same logic, through the existing recognition of opposite-gendered marriages. Recognizing same-gendered marriages neither increases nor decreases the likelihood that those feared scenarios will come to pass.
The reasons for recognizing same-gendered marriages are the same as the reasons for continuing to recognize opposite-gendered marriages. However, some of the reasons given for banning same-gendered marriages would, if officially cited as reasons behind the ban, inadvertently set precedent that could later be used to ban marriages of some opposite-gendered couples (most notably senior citizens and couples who are unable to procreate, naturally,) to cease legal recognition of clergy-joined unions or even to do away with legal recognition of ALL marriages.
NOTE: There is already a small but growing number of activists on the anti-gay-marriage side of the issue who are on that side of the issue because they are anti-marriage, period. These people are, indeed, planning to use the precedent set by gay-marriage bans as a means to end legal recognition of any marriage. If you are among those who are worried about how legalizing same-gendered marriage might affect your opposite-gendered marriage, you should be aware of this movement and also worry about how banning gay marriage might affect your hetero marriage.
Gay Marriage -- Table of Contents