@LilyWater
OP, as others have alluded to, you're trying to run before you can walk. Before you have kids, you need a firm foundation of a committed relationship. You yourself sound like you're in this for the long haul but in your shoes I'd be concerned about your boyfriend. I've seen this so many times with friends and acquaintances who got together young. If by mid 20s, there was no proposal from the guy they are either still in the state of perpetual boyfriend/girlfriend with no marriage in sight (and the woman secretly pulling her hair out at the lack of commitment which carries on even if they have kids together), or they've broken up and both are with new partners where the subsequent relationships progress much more quickly because the guy in particular has found someone he's excited about committing to.
The blunt truth is that most men take women for granted who they've been going out with that long. You have already given them everything they want on a plate (convenient sex, emotional support, someone helping them with domestic chores and financial costs etc.) plus with the bonus of no need to progress the relationship. They like the comfort of being with someone they know well and make all the right noises so the woman doesn't leave, but often boredom has seeped in because they've been with the same person for so long and they start wondering internally if "this is it". They normally dont leave until they find another woman who excites them and they want to marry, or the woman issues an ultimatum to stop him wasting her time. Beware.
That's a grim way of looking at it, and I do think it can be more complex than that, but yes, I think there is some wisdom to be had here from LilyWater. An acquaintance of mine believed that many men leap from one relationship to the next - they won't leave until someone else is in the picture. Part of the reason why the older ones do this is that they quite like having someone run around them, sooth them and look after them, and they won't leave that comfortable, yet dysfunctional environment until something exciting comes along. Actually some of them don't leave - they just want to have their cake and eat it which is why some of them find a mistress who they will also string along and never commit to.
Marriage and having that certificate or ceremony is a nice idea, was never important to me (having seen the rates of divorce). Marriage doesn't transform your personality - if you were a right shit before chances are you'll be a right shit afterwards, and with some men, marriage can make them even worse because they think that implied or required loyalty that comes with it allows them to behave in even worse way. With the security of marriage, they start to take their wives for granted and taking the piss. You can get men who want to marry, not because they love, but because they're quite possessive and want no part of your life to be truly your own, and those men are usually a nightmare to divorce.
However, what you need to consider that the reason why marriage may not be important for another person is entirely different. There is a type of peewon who is quite 'meh' about the relationship and will remain in it as long as they are not required to make a solid, legal commitment. I would be concerned that your guy, despite seeming lovely and sensible in many ways and saying that he loves you, is basically trying to delay growing up and commitment for as along as possible and you will pay the price for that. I would be less inclined to think this if your relationship was less than a year old and you were still getting to know each other, but he knows exactly who you are, hopefully, and you have been together for a really long time.