Re: Question regarding unfixed male/female pair
It is VERY likely he will spray, especially when he has a female in heat around, to ward off other males. Not to mention you have female heats to deal with which includes insistent yowling to call to mates that might be around and random intact males showing up at your house, she will probably spray while in heat as well to say to passing males "I'm ready!!". Female cats will go into heat every few weeks for months. Your intact male may also try very hard to get to your female, no one should have a six month old pregnant cat (about the age she can start getting pregnant, although it's not unheard of for it to be earlier) so every effort has to be made to keep them apart which really stinks because you want your cats to be able to hang out together but since she'll be in heat very frequently that will be hard to accomplish.
You also have to worry about your male trying to escape in order to find other mates, they are in no way monogamous so even if he has an intact female at home he will try to get to others around town. I've also read female in heat will try to escape.
Overall I think having sexually mature cats is a pain in the butt and dangerous, cats may get outside and get killed plus the danger of the male getting your cat or other cats pregnant resulting in more kittens in a world FULL of unwanted kittens. I go on craigslist and am appalled at the number of free kittens, people just don't have room. To be worthy of breeding cats IMO someone needs to have something very, very special, cats with therapy cat titles to prove they are outgoing, especially in outgoing breeds like Siamese, it proves they have the proper outgoing temperament(yes it can be done) or who have proven themselves in the show ring to be excellent examples of their breed. The last thing we need are just more "good pets" with no proof of that other than the owners saying their mom and dad are nice to them. I can find a few million of those on petfinder thank you very much.