So you would just let your kid be cruel or bully another kid because he/she is different? That's where parents and any supervising adults should come into the equation. If parents tell or corrected such instances or explain to them of such things then kids will understand things eventually (that's if these adults have no problems with same sex couples). Wouldn't you correct young kids of any bad behavior in the first place? Teens are quite more complicated because they have their own mind. Kids, they reflect what things are being taught and told by the adults. If their parents and adults surrounding them tell them being gay or in a same sex relationship is awful/bad/disgusting, that's how they will perceive it. That will be what kids of same sex couples would most likely experience.
I don't have kids but I've got young nephews. When the eldest started learning how to curse at the age of 2 (he heard it from adult neighbours), we corrected it. I haven't heard him curse again since that week (he's 10 in Sept). With the youngest one, up to now he still drinks milk from the bottle and he'll be 6 in August. No one was telling him to stop coz my sister & brother-in-law are too busy to stop him & it's one of their way out off his tantrums sometimes. That's two different instances where adults were in control and not in situations with kids.
Like I said above, this situation will be more acceptable in the future. You can already see that although slowly, gay marriage is being accepted by society. It's mostly the old generation and religious ones who have problems with it The video I posted above shows that even young kids can accept it.