The discrimination we face as same-sex couples is based on sex, not gender. The terms are not interchangeable.
Sex identifies the biological make up and difference between the male and the female.
Gender is constructed socially and identifies the relationship between men and women. Many people support the idea of marriage. Those who don't seem less concerned with the language. They either don't understand the issue as one of equality, don't think marriage is a good idea for anyone , or simply dislike gay men and lesbians. However, doing so sacrifices the clarity of understanding that the sex status of gay men and lesbians is fundamental to the marriage discrimination we experience.