Why do cats come to visit my two spayed female cats?

Kitty is probably defending her turf, marking it so these visitors know this is HER territory. Even spayed/neutered cats can be very territorial. Covering the windows so Kitty can't see the visitors, getting a kitty pheromone spray or diffuser (D.A.P. is one brand) or getting some anti-anxiety meds from the vet might help. Enzymatic urine cleaners, the kind that you saturate the carpet with and let sit 24 hours, work best to get rid of cat smell.

Your Kitty IS marking her Territory...-and your cats are probably just a couple of "Babes" in Feline High Society... :)

You didn't say if Kitty has seen a vet lately or how old your cats are. Cats can get urinary tract infections like humans can. And yes, it can be a territory marking thing. Try using Feliway, either the spray or plug-in; the cat facial pheremones can help calm cats under stress.
Cats are social, but the peeing is their "fence"; it says to the visitors, "This is my territory, keep out." Keep the pee'd on places clean--don't use anything with ammonia--there are several products on the market that clean and deodorize cat urine.
You might consider putting like a poster or foil, anything that blocks the sight of the visiting cats from your cats on the glass doors or windows. There's also various cat repellents, like one called SSSCAT! that uses compressed air; harmless, but plays on most cats fear of the hissing sound.
The other cats could just be checking out the 'hood, and decided to bully your cats by coming around. It's their form of intimidation.

It has nothing to do with the male cats being attracted to your cats.Cats will usually hang around where other cat's do, also if your leaving any cat food out that may attract them as well.

I think them cats from the neighborhood got the hots for your kitties ;]