And if you asked the population of people who have actually been brought up by gay parents whether they were happy with that situation, my guess would be that the vast majority would say "yes".

Is there enough data on that to be statistically valid?

Are a truly random sample of children brought up by gay parents? I'd expect some large selection bias there.

Just ask people `if they'd have been happy being brought up by [insert any religion, or lack thereof] parents, the vast majority would say "no"'.

How could that question not reflect respondents prejudices more than anything else?

If instead you ask people who actually know what the hell they are talking about, q.f. people who were brought up by gay parents, you will find the vast majority to be quite happy about it.