What @Bastethotep said. There is indeed something of a mythos around the USMC; it’s useful for a fighting force to have this, sure, but just don’t believe the myths too much. These people are real human beings, after all.
This was a training exercise. A minor set-back. It probably didn’t bother the USMC members concerned all that much, why should it bother anyone else? To say it was because some USMC are gay or transgender is insulting to all, not just gay and transgender people. The quotee ought to be pleased that the Royal Marines (some of whom are also LGBT, of course) are on the same side. After all, the UK and the USA are usually allies.
Why not offer a hurrah for our lads then, instead? The USMC is based on the Royal Marines, after all.