It may sound like I am trolling but it means "No or Maybe".
Don't worry about this. It doesn't mean he never will. But I am sure he has never thought about marriage in his entire life. Guys have other things on their minds.
If you are serious about marriage (and this is not some test) it's a good thing you have let him know. But don't force it. I think for us marriage is sex like for women. It doesn't mean she won't ever have sex but she just needs a little more time. So when it comes to marriage give him that time.
It means 'no' and likely it's because he hasn't thought about marriage and likely won't for a long time. I'd it him in the 'not marriage minded' category and walk off if that's your ambition in the near future. Men are not on the same timetable as women on these things. A girl could waste a lot of her fertile years and youth on a guy who hasn't a clue that he is taking up those things she can never get back.
It means, drop him. Ain't nobody got time for a guy who is turned off by the possibility. It is not pressure to marry, but if he can say no, drop him like hot coal. Some guy out there is crazy about you and will jump at the possibility.