There are many reasons why women cheat on their husbands and boyfriends. A big reason is that the men in their lives start taking the women for granted. They stop telling them how beautiful they are or how much they desire them.
Guys, women want be know that regardless of how long you've been with them, that they're still sexy to you. When you're with one, tell her how beautiful she is. Tell her how pretty her eyes are or what a nice hairstyle she has. Tell her how much you like her shoes; not because you have to but because you want to.
When you're having sex with her, tell her how good she tastes. Tell her how much you love being inside her, any part of her. Tell her how beautiful her lips are, both pair of them. Tell her how soft and smooth her skin is. Get nasty and tell her what a good fuck she is.
A woman, any woman, needs to know these things. They give her confidence and self esteem. That, in turn, will mean better sex and love for you. Oh and mean what you say. Fakeness is never a good thing. Don't over compliment but compliment, nonetheless. If you don't tell her all of these things and mean them, then she'll look for someone who will. I guarantee you, I will be that someone she's looking for because while you're wondering where she is, I'll be the one sexing her....repeatedly.