This is true, but for the TWIC card at least, the face of the card does not list the gender marker, unlike other IDs like passport, MMC, and drivers license where the marker is below your name - TWIC just has picture, name, and expiration date, and the TWIC is what you use to get in and out of the ports, so at least that is one interaction that can go smoothly if you can't change the marker.
Also, I just renewed mine a few weeks ago and it was all by mail, I did not have to go in for a new picture. In the past when I went through the name change process (and needed an updated picture), I had to request a new picture appointment and then I had to double check with the NMC that they would use the updated picture for my MMC when I got that renewed. I had to call them and talk to a representative.
Another thing is that with the new style MMCs (single page), the ship's I have been on since the update, the Captain's have been posting them opened up to the credentials page on the bulletin boards. This has your name very small at the top and no picture or gender marker, so can alleviate at least a little stress if you are worried about visitors seeing the board and outing you. If the Captain posts the folded ID page, I think its a reasonable request to ask them to post the open page instead, this also helps if your picture is a little outdated.
