I was in a similar situation. On Sex and the City (yeah yeah bite me) they stated that some guys have their "light on" like a taxi. They can drive around for years and years picking up people and dropping them off but until they have the light on, or are ready for marriage, it doesn't matter who you are or how long you have been together.
My boyfriend has the "light on" and we are able to openly talk about marriage (even though we want to wait a while longer). I understand why he wants to focus on school (that's what I am doing now) but I still don't have any issue with discussing our future with my boyfriend. I also don't feel pressured when we discuss it. You shouldn't date someone if you don't think you will marry them later on. This of course, doesn't mean to go out and ask everyone if they are ready for marriage, but if I ever started to feel like I really DO NOT want to be with this guy, I would give it a week, see if my feelings change, and if they don't, move on. It saves a lot more pain in the future.
I'm not good at beating around the bush or sugar-coating things. My responses are never personal vendetta's on your emotional state. It is what it is, whether you choose to accept it or not. <3