It depends on the woman.
Some women are VERY loyal. Some women are just sluts or can't be trusted, EVER--PERFECT future -x-wife material.
I think you can usually tell right away if a woman can be trusted or not, based on her interactions with other people, especially other men. If she's flirtatious, etc, you will have trouble. If she has a lot of "guy friends", you have trouble. You get the picture. Don't ever fool yourself into thinking she can "change." If she's a slut, that's who she is.
If you want to make a loyal or loyal-potential type woman MORE loyal to you, make it KNOWN through your body language/life/interactions with everyone you meet, that you could EASILY GET a newer, most likely HOTTER woman than her, but that you stay with her because you "like" who she is. (Of course, never come out and SAY you could get a hotter woman). This is what has worked for me.
ALSO, never show that you are paranoid or worried that she is cheating--this is fuel for the fire or will at least make you look like a wuss who has no confidence.