Whether that's relevant depends on your values in the first place: are you a harm-based consequentialist?
As it happens, yesterday I took a survey ("Argument Evaluations") on YourMorals.org which asked exactly that question ("how relevant is the argument that 'children raised by gay couples may be harmed' to the morality of gay marriage") and you will be unsurprised to look at the results and see that people differ on what arguments are relevant to the morality of gay marriage:


(Green is me.)
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.