I see it as politics. Every President is expected to offer a kind word at the advent of this "gay pride" month. If they didn't, it could be blown way out of proportion politically to try to sink their chances of re-election. He might have tweeted the message holding his nose. Politicians have to play politics.
I don't think Trump is "pro-gay". Honestly, I think Trump could care less. But if he does have any feelings towards the matter, I'm willing to wager that it is more along the lines of being "pro-human rights". Every man and woman has a right to live however they choose. However, we know that concerning God's eternal city...
Revelation 21:27
27 And there shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie: but they which are written in the Lamb's book of life.
The wicked will have no part in the New Jerusalem.