For one, being childless is a choice (mostly, especially since adoption is a possibility). It's indeed OK to have different opinions for what how laws apply differently to people based on their choices. Being gay is not a choice, it is rather similar to race/ethnic background, and it's generally not OK to have laws that treat people differently based on something like that. I'm sure there are more nuances to add, but it seems to me that makes it quite a different situation.