Alright, so I'm going to say it now that I'm a tad biased with this one, but for good reasons. I've always found that marriage plugins go really well on servers, due to the fact that it keeps players invested socially and adds a benefit to those who do socialize. I've also seen it benefit many servers in terms of gaining a loyal following, which is always a good thing.
If it was added, there'd have to of coarse be some configuration that'd need adjusted depending on the preference of the staff. I personally recommend if there is a priest option to make it so there are no priests, and players can marry each other without a 3rd party. Another recommendation is to allow single-party divorce and offline divorce (single-party meaning only one has to consent to it, not both. Offline is good with it because then they can divorce even if their partner is offline, which is great for if your partner leaves or something like that). I also recommend allowing same-sex marriages if you decide to add in a gender plugin to compliment it, which isn't at all required.
Again these are just my thoughts, wanted to know what everyone thought :3