In this case, marriage.
Inevitable disclaimer: I like some gay people, OK?
I like about the same proportion of gay people I know as straight people I know. My brother is gay. My brother in law is gay. The best man at my wedding, my best friend at the time, was gay.
I absolutely believe homosexual men and women should be protected by the law from any form of discrimination on the basis of their sexuality. I believe the law has no place in people’s bedrooms, provided what happens there is between consenting adults. I think homosexual domestic partnerships should be recognised and given some protection, for example in matters of life insurance and superannuation.
However, I do not approve of homosexual behaviour. I believe it is harmful for those who indulge in it. Accepting that it happens, loving some people who do it, does not mean I have to believe it is a good thing.
States should not be pressured into calling homosexual domestic partnerships ‘marriages.’ They are not marriages.
Some non-religious reasons for this view are discussed on Secular Right.
That the meaning of words should not be arbitrarily stretched to the point of emptiness for political purposes is just one reason. Here’s an excerpt:
There really is a slippery slope here. Once marriage has been redefined to include homosexual pairings, what grounds will there be to oppose futher redefinition — to encompass people who want to marry their ponies, their sisters, or their soccer team? Are all private contractual relations for cohabitation to be rendered equal, or are some to be privileged over others, as has been customary in all times and places? If the latter, what is wrong with heterosexual pairing as the privileged status, sanctified as it is by custom and popular feeling?