What are you going to do if he starts spraying that wonderfully smelling un-neutered male feline urine that will stink up your home so fast you won't know what hit you? Will you still think your un-neutered male is the sweetest kitty around and shouldn't be neutered?
As others have said, there are millions of cats with your cat's beautiful looks and personalities to match that get killed every year in North America. Do those homeless cats a favour and try to get the homeless female spayed, and your male neutered. You would actually doing both of them a favour. AND it will probably solve your cat meowing problem
TokyoParrot has given you some great ideas. Here are some more to assist you in trapping her if you choose to do so:
1. Cover the trap with a towel or blanket so the cat feels more secure.
2. Us smelly food, such as sardines to attract her.
3. If she is really hesitant, place the food just outside the trap door and over a few days, slowly move the food towards the back of the trap.