Too early to think about marriage?

I've been with this guy for 4 months I'm his first girlfriend and I'm absolutely amazed by how he treats me and how this relationship is going guys don't stick with me well and usually try to use me for sex so this is a shock for me but I love him so much and I want forever with him and not be heartbroken again I want to marry him in the future cuz we're only 17 but I hope we last that long.