They're already in the Democratic Party. The Democratic Party is pretty conservative. One third of Democrats still oppose gay marriage, even today, that's one third of the party that would be banned from this site according to the rules if they ever said their opinion on that issue.

You seriously need to reconsider the reality of political ideology in the US today.

Many minority communities that Democrats want to reach out to hold very bigoted and hateful views overall. I know it's easy to pretend as long as we get all the "good guys" out to vote that should do it, but the reality is that hate and bigotry are in all populations, and politics doesn't eradicate it. Political reality requires getting bigots on your side, and Democrats do it all the time, they threw gays under the bus as often as needed until gays and their allies shifted the conversation to where it was no longer practical to do so.