No doubt, this is a deal-breaker as well. If you enter into a monogamous relationship, I definitely would expect myself and my girlfriend to hold up to such a standard. Infidelity (at this stage in a relationship) is paramount to betrayal at the deepest level IMO.
If you want to cheat on somebody, please break up with them before you do so. Nothing erodes confidence and emotionally ruins a person like hearing they have been cheated on. My neighbor is 25 and her ex-husband is a serial cheater (happened before and while she was pregnant with their kid) and from what I know she is going through a very tough time in life still living with her mother who does her best to provide the emotional support that she can.
I saw him outside her house today, and I couldn't think of what a scumbag he is and how I would be doing the world a service if I walked over to him and beat him into their lawn's landscape. Betrayal of that magnitude is just heinous IMO. Makes me sick to my stomach to just think about it.
As far as drug use goes, as long as my girlfriend doesn't surprise me with some sort of meth addiction upon moving in, I'm okay with whatever as long as she is candid about it. Though, she probably won't be making much money at a job that doesn't drug test, but that's where my job as a lab techie for an oil company comes in So, drug use wouldn't bother me, but drug abuse is a whole different story.