For example, in order to get a driver's license, it has been deemed that a person must be of a certain ageand have to pass certain tests as deemed necessary to safely drive in the state, all of which are valid restrictions because they are done so with the state interest of ensuring that competent people who know the rules of the road and how to drive are the only ones driving in order to try to reduce the amount of traffic accidents. The restrictions are not arbitrary or "traditional", but rather backed by knowledge about what and who causes road accidents most often, and they are updated (in some places, not as quickly as some would hope, I'm sure) as new information becomes available. However, it would be against equal protection standards to say that people who have no hair on their body are not allowed to get a driver's license because they are not trusted by other people or that they are in some minority of people or that they traditionally couldn't get a license (and, yes I am making this up, it is a theoretical example). The licensing authority of states still has to meet equal protection requirements, no matter what the license is.
The marriage license, however, is inaccurately named though since a person can get a marriage without the legal paperwork that goes with it being legally recognized by the government, unlike pretty much every other license there is. A person cannot legally drive without a driver's license. If they are caught driving, they will be punished. A person cannot practice in those professions that are licensed without some sort of license to do so. If they are caught, they will be punished. The same cannot be said for a marriage. A person can get married and live a married life with whomever they wish, as long as they are not trying to get the government offered benefits/rights that go with the marriage without getting into any legal trouble for doing so, in most states. And in those states that might try to get them in trouble, for something such as polygamy or cohabitation law violations, they could easily fight it in court and most likely have such laws overturned.