We do it all the time. Don't drink and drive. Don't drive over the speed limit. Don't yell fire in a room if there is no fire. Gender based bathrooms.<quoted text>
Since when do we curtail people's rights based on their participation in risky behaviors? Heterosexual couples can marry even if they engage in fireworks manufacturing, skydiving, shark wrestling, and even smoking unfiltered cigarettes while drinking gallons of vodka. And, heterosexual couples engage in the same behaviors that you say should preclude homosexual couples from getting married, but you don't support banning such heterosexual couples from marrying. So, engaging in risky behaviors is clearly not a valid reason to ban anybody from marriage. Nice try.
It is only by heterosexual couples that the next generation is brought into the world. Can't be done by homosexuals naturally.
Society gains nothing from same sex marriages. They can't procreate on their own nor can they model parenthood in a balanced way. Children need a female mom and male father. These are the best models for children.