I always understood the male cabin and catering crew were deliberately selected to service the gay cruise market
Not so. I know a lot of Central Americans who have worked as cabin and catering crew on cruise ships. One of them told me of one idiot on the ship he was working who was propositioned by a gay man, decked him and was off the ship at the next port of call. Most of the crew, however, really like working on the gay themed cruises as the passengers tend to tip particularly well and, for the most part, are respectful and well mannered (i.e. treat staff as human beings and don't make advances to crew members).
Crew members need quite a lot of training and induction and serve on cruise trips with all kinds of theme. It would not be cost effective to look specifically for crew members of any particular type. The requirements for cabin and catering crew are mostly focused on speaking enough English, knowing how to deal with people and passing medicals of various kinds. Agents liaise between applicants and the companies who take fees from both, arrange visas and first flights to the ports where the new staff board their first vessels. Just to add that the hours are very long and the work hard and boring. Injuries or work related sickness are treated generally up to a point, but there isn't much in the way of employment law at sea to protect workers.