I've been dating the same girl for 4 yrs and she wants to get married, is it safe to say she wont cheat on me?

I've been dating the same girl for four years, on and off at times, but pretty steadily for the last year or so. Lately, we've been talking seriously about engagement and marriage, and she wants to be engaged ASAP! I am all about it, but can't seem to let me guard down. I keep telling myself that if she truly wants to get engaged asap like we've been talking about for a year now, then she obviously isn't interested in other guys! Am I stupid for not relaxing?!