Denying them is a violation of religious freedom (civil and religious marriages are two separate institutions).
Marriage benefits (such as joint ownership, medical decision-making capacity) should be available to all couples.
Homosexuality is an accepted lifestyle nowadays with most evidence strongly supporting biological causation.
Denying these marriages is a form of minority discrimination.
It doesn't hurt society or anyone in particular.
The only thing that should matter in marriage is love.
The number of child adoptions should increase since gay couples cannot pro-create (although some might see an increase in gay adoptions as an argument against same-sex marriages).
It encourages people to have strong family values and give up high-risk sexual lifestyles.