I think the decision whether to marry is comprised of two parts:
1-you realize you've reached a place where you know the other person well enough to fully trust that your happiness and needs can be sufficiently met by the other person and the relationship as they all currently are, and so can theirs. Also, you have a reasonable basis to believe things will not only sustain but naturally improve/deepen over time, not as a requirement, but as a natural byproduct of shared experience and trust.
2- Once that realization occurs, you rely on it to simply decide that you choose (and will always keep choosing) that person and that relationship and choosing to do whatever is necessary to prioritize the health of that for the rest of your life, no matter what (barring freak, unforeseen, unknowable circumstances), because you trust it will always be worth it.
I think a lot of people get married because they think they're supposed to, and they hope the act of it will suddenly bring steps 1 and 2 into being. This is a mistake. However, if you realize steps 1 and 2 actually do exist, I think you probably should get married (since you're already considering it), because certainty is essential to the ongoing balance of trust between the parties.
It's like saving a level in a video game. Let's say you and your partner's decision making process got to the same place--you both have completed levels 1 and 2 above. If you don't save the level (aka get married) one or both of you could end up repeating a previous level because lack of certainty in the situation can make doubts creep in about whether the other person is really in the same place with you about the relationship, causing needless stress.
I say one or both of you could end up repeating a previous level, but more likely it will be your NT partner who feels the lack of certainty the most. This is because it is truly a leap of faith for an emotionally driven NT person to simply believe their aspie partner still feels the same about them in the face of behaviors the NT's entire life experience has taught them signals the exact opposite. Make no mistake, this is hard for your NT partner to do and requires a lot of consistent self mastery to view things through the proper aspie lens and keep the doubts at bay. And since it can be difficult for aspies to express their feelings and emotions about the relationship in a consistent enough way to naturally produce a feeling of ongoing trust and security in their NT partner, if you feel you've reached levels 1 and 2, save the progress by marrying and level up!
I would never say marriage is a required essential for everyone or that no AS/NT couple could make it without taking that plunge, but I will say this--it provides a baseline level of certainty that mitigates the natural human tendency to worry the other person's feelings have changed/digressed. If you're truly in the right relationship with the right person, marriage will be freeing and help propel the relationship onward in a positive, fulfilling way.