I understand how you feel.
I find the marriage thing really difficult.
I read somewhere (oh, that book - the 'not into you' one) that if a guy doesn't want to marry you, he's not, erm, really into you. I'm not sure if I hold with this.
I've known people be together for many many years and only marry when they got to their 50s or 60s, because they just decided it was time, and people who have stayed together a long time, not married, and then decided they rather wanted to marry someone else instead. (DP did this...left long term partner he had never married, and decided to marry me! Oh dear
)
It does sound as though you have massively entwined lives and are both very committed though. Which is more than a lot of people have, so you're doing ok imo.
Personally, and this is all I can offer really - if I am with someone and I love them very much and we get on well, etc etc, yes I'd like to be married. We haven't yet as financially it's going to be a big commitment and a big change, but knowing we have agreed to marry one day is important to me.
I think that's just because it's culturally driven into us that that's what you do if you love someone. Nothing more, nothing less. And as you say, having to explain all the time to other people that you're not married is very tiresome and sometimes it's nice just to fit in, isn't it?
I've already had to remind strangers that I don't have a husband yet, he's my partner, and they still insist on saying 'your husband'...we only got engaged in February.
I hope you manage to resolve it.