Gays should definitely be able to marry. Homosexuality is natural; homosexuality appears in a high number of animals beside humans.
Homosexuality is not something that just 'popped up' suddenly - it was well known and occasionally practiced before religions such as Christianity and Islam. For example, some Ancient Greeks and Romans were know for homosexual tendencies.
Marriage provides benefits, while Civil Union does not. Your religion does not affect your sexuality and vice versa - there are many religious homosexuals.
Abuse because of certain differences is common. Three hundred years ago, African people were very often enslaved, abused, forced to work and sold, as if they were belongings and not humans. A hundred years ago, women were not allowed to work, often faced oppression from their husbands and were rarely allowed to work to make a living. Today, it is the homosexuals that are getting picked on, and soon hopefully this issue will become a thing of the past, as society becomes more understanding and open.
Just because gay couples can marry, does not mean that the world of 'straight' people will suddenly change. It's not like your children will get bullied because of heterosexuality, or that the Rapture (should God exist) will happen yet nobody will be brought to heaven due to acceptance of homosexuality within society.
Using Religion to prevent homosexuals from marrying is morally wrong. God never said anything about homosexuality being a sin. Man and women are the means of survival of human race, but considering that there are over seven billion people on Earth at the time of writing this, sudden extinction due to a lack of heterosexual couples is simply impossible.