The tone of your letter suggests you haven't made your position sufficiently clear to your boyfriend. Yes, you've had discussions on marriage and kids, but so has every other couple in the world - even 13-year-olds at the hand-holding stage chat about marriage and kids! But nine times out of 10, these discussions are purely theoretical - moments of wistful gazing towards a distant horizon. So just because you've discussed marriage and kids with your lover, doesn't mean you've driven home the message that you want to walk down the aisle THIS SUMMER and have a baby nine months later.
Perhaps he mistook your little discussions as little mutual affirmations that you really like each other and want to stick around some more. Remember that for a guy, starting a family is something he's physically able to delay (virtually) forever. There's no tick-tock of the biological clock for him, lucky sod!
So your boyfriend has probably given no thought to the fact that you're looking to your nesting years with some urgency. So tell him. Sit him down and explain exactly how you feel about him, how you hope your relationship will last forever, how you're ready to commit and how you're very keen to start a family soon. Don't let him fob you off with nonsense about marriage putting relationships in severe danger. If he repeats this comment, point out to him that while it's true that some partnerships flounder after tying the knot, many others are wrecked by commitment-phobes who never make it to 'I do'.
Perhaps you could mention that in your view, the bonds between lovers become stronger when they embrace new challenges and responsibilities. And though there are moments of strain and hardship when children hit the scene, a good relationship will only get better and more fun after kids. I hope your boyfriend takes your talk seriously and that he responds thoughtfully and (after some initial reluctance perhaps) positively to your idea of marriage and ankle-biters. If not, you really should consider ending this six-year relationship. I know it would cause immense pain but living like a student no longer suits you, it feels like a thin and temporary existence, so please don't accept more of the same.