Here's a quest hook for you though, no matter what kind of game you run: an NPC proposes to one of your player characters.
There are several ways to go about this, I've gone over some below. In all cases, make sure the person proposing is an interesting character with ties to your world - this raises the stakes no matter the outcome. Use your favourite NPC.
"The Gritty": Marriage is a patriarchal institution by which men can trade in their daughters for gold, land and oxen.
Adventurers, assuming they survive at least a session, have more gold and treasure than they know what to do with. As your PCs gain in wealth, they will garner interest as potential suitors. What enterprising fellow wouldn't want to marry his family into that? And if they die on their next excursion, his daughter will be a wealthy widow.
Of course, if the player doesn't die immediately after the reception (or during, go full Game of Thrones), they'll have gained access and influence in one of your world's factions, however minor, with ties to NPCs and some small slice of your world.
Or, with marriage as more of a commodity, maybe treat it as a prize, the "treasure" at the end of a quest: with the dragon slain, the barbarian king acknowledges your strength, and gives you his heart and the service of he and his warband - as is tradition.
Do bear in mind, those of you who love your "gritty, realistic" worlds, that there are a lot of popular myths about medieval marriage. F'rinstance, women marrying young was not a thing in medieval Europe outside of royalty - who, let's face it, were just generally pretty messed up anyway.
(A wife was for housework and childbearing, and she couldn't do either effectively - especially not the latter, which risked the life of both mother and child - until she was at least twenty-one or so. So things were still shitty for women, just not quite child bride levels of shitty.)
|I think I actually watched this movie|
Check your players' CHA scores. Chances are they are, by and large, more attractive than the average person. OK, CHA doesn't mean hotness, but they're certainly more interesting than most. Why wouldn't someone equally interesting, or perhaps even more so, be intrigued? The court wizard, perhaps, or a faerie prince.
Make the proposal come from someone powerful so that both rejection and acceptance will carry with them a cost and a benefit - the gaining of both new influence and allies but also responsibilities, or keeping one's freedom at the cost of incurring wrath. Rejecting a demon queen is campaign-changing stuff; nobody will remember the time your fighter turned down a starry-eyed milkmaid.
(Unless, of course, a djinn or demon hears the milkmaid's lonely sobs, and offers comfort, or even revenge... Kind of a dick GM move imo but sure, fuck their shit up.)
"The Politician": Marriage is a means of social positioning.
This is for those Masked Ball kinds of games. Maybe someone has something your player wants, and they'll happily give it up in exchange for their hand. Maybe your PCs already have some influence, or gain some through questing, and that makes them desirable.
You'll need a succinct but tangled web of NPC motivations, and players willing to investigate and learn. Whose family wants what, and who in that family wants something different? Brush up on your Shakespeare, and mire the whole thing in ulterior motives.
"A Rose by Any Other Name": Not marriage at all, but the same idea.
Of course, nobody necessarily needs to propose, it's just a clear, immediate and dramatic version of the real plot hook at work here - an NPC puts themselves on the line and offers to start a relationship with a PC.
Take that however you choose. A flirtation leading to a possible one night stand, even a business proposition - anything that will make the player's situation more complicated than it was before, no matter how they react.