1. Marriage is a civil right granted by the government.
2. Everyone should have the same civil rights under the law, regardless of their sexual orientation.
3. Same-sex marriage hasn't been recognized in every state, so it is not settled.
4. Same-sex marriage has come to symbolize the greater struggle for acceptance and legal equality for LGBT people. It raises the basic question of whether it's OK to treat people differently because of the sexual orientation and whether being gay is wrong/a choice. So, the fight for SSM has advanced acceptance and equality for gay people across the board.
5. The US still has tons of homophobia and mistreatment of gay people so we have a lot of work to do in general to make acceptance of homosexuality a social norm. We've come a lot way in the past 10 years, but there are still anti-gay hate crimes, conversion therapy and bullying of gay kids going on. We can't accept second-class citizen status on any issue or be complacent even when we've had some political success in recent years. As if it's OK to be homophobic or legally discriminated against in some states. It's never OK and we shouldn't have to take it any longer.
I have a reasonable counter here.
1. Marriage and the family was instituted before the concept of human government. Thus goes beyond civil rights. (Look at Genesis 2-4 on this)
2. Those who want to change that are trying to undermine cultural and societal norms that have always existed. Thus the burden of proof to change roughly the whole of human history is on the ones trying to change that incontrovertible fact.
3. The Founders to a man agreed with the Biblical view of marriage and family life and shouldn't we at least consider the wisdom of the founders even if many of you want to cast off the Bible and its role in American jurisprudence.
4. For the courts to undermine the will of the people specifically expressed through their legislatures or through voter referendum to codify traditional marriage as the only acceptable marriage in said state is a stain on the very courts themselves
5. The state's who had bans on SSM who had them overturned by federal courts (yes I'm looking at you too California and Prop 
should have said bans restored and all said "unions" voided from the state records at minimum.
My counter-counter.
1. Argument to antiquity. The ancients could be wrong -- very wrong. Slavery and aggressive war used to be acceptable; they are now recognized as horrible crimes.
2. If love is a good thing, and some people are capable only of love within their own gender, maybe we would be wise to accept marriage based upon love and without overt exploitation if it is same-sex. If the government can promote marriage among heterosexuals for benign reasons it can accept marriage among homosexuals for benign marriage.
3. The Founding Fathers were all white male property owners. They were not radical reformers. They accepted what they liked about the old way of politics in the British colonies as the norm for subsequent times and rejected a capricious, despotic King. White supremacy, male domination, and the institution of slavery would remain. Government would represent property owners and certainly not paupers.
We know the consequences of slavery. Women got the vote and are not going to give it up. The poor have as much right to representation in government as do the rich.
4. Re;publican state legislatures in Michigan and Ohio did not put same-sex marriage on the ballot as initiative and referendum during the 2014 election. They could have, but lacked the political courage to risk offending a big part of their constituency of Christian Protestant fundamentalists.
5. Mistakes of law in good faith by officials are adjudicated to the benefit of individuals unless gross wrong results.
