“Why? The US contains just as many anti Trumpets, certainly more in absolute terms. The government invited him and these opposition MPs are in the minority and don’t reflect public opinion, which supports the state visit.”
Why go? And risk that the welcome is not fitting the President of the US. Or some other diplomatic slight.
The risks outweigh the return, by alot.
You are always going to get anti-American trolls turning up to protest a US Presidential visit in a foreign country. The only countries were that wouldn’t happen are probably repressive dictatorships that would crush any protest with force of arms, and which no democratic leader worth a damn should be going anyway.