They figure they have rock-stable support in the low 40% range, roughly what the GOP (a/k/a GOPe) has. To secure an election win, they target specific smaller groups that have very high one-issue vote decisions.
The judgement here is that the overall LGBTQetc population AND their sympathizers are worth a few percent, and will be stimulated to vote by attention to any issue in their camp, even an extreme one.
It's cynical, but it's their call. It worked pretty well, although the rhetoric was more respectively cloaked, for Obama in his elections.
But they didn't figure on Trump.
Approximately half of that 40% are GOPe haters, what used to be called yellow dog Democrats (Before I'd vote for a Republican, I'd vote for a yellow dog").
Why they hate Republicans is complicated, but understandable. Ronald Reagan understood those people, and it is beginning to appear that Trump does, too.
If that 40% base bleeds 5% (never mind ten or fifteen), no Democrat can win a national election.