This thread will give you some very good reasons on why you need to be married if your partner dies.
It's issues such as, if your child needs hospital treatment, only the mother can sign the consent form, regardless of whether her partner is named on the birth certificate. If you are married, either of you can do it.
If your partner is ill, then you can not sign the consent forms, his/her parents have to do it,
Likewise, if they die, as a partner you can not sign the death cert, but as a wife/husband you can.
If you are married and one of you dies then joint credit cards would be closed, you wouldn't get a grant for the funeral, all belongings would go to his/her family, you would have no claim on their pension, you can't own their grave, or order the headstone.
If you have children but aren't married and you die without a will then custody automatically goes to the mother's parents - if the father has an awkward relationship with them it could be a nightmare
I think that's reasons enough to get married, don't you?