Yes, they're supposed to be celibate and yes, there are "short time" monks and "long time" monks, staying at a wat anywhere from a week to several years. It's a fluid arrangement. They do not take lifetime vows as Catholic monks do. And yes, the teenage novices are often just interested in getting an education. Thus there is quite a bit of "activity" among the gay novices, from what people say. The OP's experience is not unique. Several such reports from Laos and elsewhere.