A good male sub is hard to find, and if you are, and she appreciates and wants that, then hopefully she will make the decision to formalize her relationship with you, a bit more.

But... you are no different than any vanilla suitor who wants to marry his girlfriend- you ask, but it is her choice.

And there is much more to marriage than this- if the BDSM aspect of your relationship ended, would you still want to be married? If not, maybe then you should just be collared instead.

If you were to get married, presumable you would want to have kids. Let me tell you- women often completely shut down sexually when they have young kids. And they are often "touched out" and the last thing they want to is be licked (or even hugged, sometimes). And if they are breastfeeding...

Last, your girlfriend may want to have an open relationship. You cannot change this. She may change over time, but it is completely up to her, not you. If this is an issue, you can be patient and loving, and things might change in a year or 10 years, but they might never change. So be ready.