I think only you can know what works or doesn't work for you, and everybody has his own way of thinking when it comes to this kind of stuff (your "friend" included).
What your question made me wonder is, what is it about being married that you'd have that you don't have now? If you and he have a good relationship and love one another, the only difference (and it would be a big one) would be whether you all live together and try to create your own "bad version" (apparently) of the The Brady Bunch.
If he, himself, knows/thinks the situation would be too challenging, or something he doesn't want; you'd be entering an already challenging situation with a giant disadvantage. By virtue of the fact that he doesn't want that situation, I think that pretty says he'd be the wrong person to sign on for it.
I think you need to ask what is it about marrying this one person that you want or need, and that would make you think changing what you have together now would be worth it. Do you want more kids, for example? (If you do, I don't think he's the one to stay with.) Is whatever love you have for him (if you have any) "disposable" enough that you feel like you can find an equal love with someone else? Does he love you the way "the right person" would love someone, because sometimes not wanting to get married means the right kind of love isn't there in the first place. On the other hand, is he someone who loves you so much, loves his own kids so much, and knows how much you love your own kids, that he thinks how things are now is the best way to keep things as they should be for all involved?
I think the "'blank' or get off the pot" thing is "legitimate" for a woman in her thirties who hasn't yet had her family; but I wonder if it should apply once both people have their families... I guess I think you think seriously about the quality/nature of the love you have between you, and all the things about marriage that you don't now have, want, and why. Then follow your heart.