Quick and easy talking points when arguing for Marriage Equality:
Opposition: Marriage is defined as the union between a man and a woman.
Equality: The definition of marriage has changed throughout the course of history. Biblical marriage supported polygamy where men could take on several wives and concubines. Marriage has historically been an institution where a man would be possessive over his wife and she would be subservient. Today we see marriage in the context of loving and financially sound relationships, there is no legitimate reason to hold back on allowing this option for all interested parties.
Opposition: If you open up marriage, others will demand it as well and society will tumble.
Equality: Marriage is currently available for two people and gay marriage activists only seek to continue this tradition and allow 2 people of the same-sex to have this right. This line of reasoning by the opposition is a scare tactic to suggest that this paves the way for polygamy, beastiality, and pedophilia. This is far from the truth since those are illegal and not consensual. Two same-sex individuals committing to each other is vastly different.
Opposition: Why can't you just settle on a domestic partnership or civil union and leave marriage sacred?
Equality: Domestic partnerships and civil unions are inferior to the status of civil marriage. Marriage provides full benefits while civil unions lack in hundreds of benefits and rights that are afforded to heterosexual couples. Marriage is also a universal concept, if a same-sex couple is civil-unionized one place, and they move to another country that does not have civil unions, their partnership will not be recognized. Marriage is a concept that is understood internationally, and LGBT people should not be given second class citizenship.
Opposition: My religion is against same-sex marriage.
Equality: There is no reference in the bible to same-sex marriage. That being said, everyone is entitled to their religious beliefs and each and every house of worship can decide whether or not to honor same-sex relationships. Religion however is separate from government and gays and lesbians should be recognized under a contract of civil marriage.
Opposition: Two men or two women cannot procreate and create a family.
Equality: There are also infertile straight couples, and those who marry and do not want children. Providing government benefits and a ceremony to honor two people who have committed to one another has nothing to do with procreation. While it is true that gays and lesbians cannot procreate, they can choose options such as insemination, donor sperm/egg, and adoption of children.