I love romance. It's a wonderful thing, brings about powerful emotions, and adds lots of depth to characters while naturally adding tension and drama to every situation because it is going on. At least, that is, when drama is done right. You need look no further than Season 4 of Arrow and all the memes that came out of it to see what Romance done wrong looks like. Still, it's a good thing to have, and if you have a game with enough story to have room for it I'd recommend having it in there. Not just allowing it for your NPCs, but for your PCs as well. And by that I mean PC -> PC relationships and PC -> NPC relationships both.
However, there are some things you need to know going in. Otherwise you could have a mess on your hands.
First, It's Not For Everyone Or Every Character
Before introducing romance to a game, you should first make sure that your players are - in general - ok with the concept of romantic relationships being portrayed in the game. Most groups have no problem with it, but some don't want to deal and that's fine.
Beyond the general ok, you also want the ok of the individual player you think will end up in the romance - or you intend to try to start a romance with (regardless of if PC or NPC) - that it's something that they are 1) interested in playing, and 2) ok with having at the table. Some people are all for it. Others are fine, but prefer to keep it private. Still others don't want anything to do with it. Romance just isn't part of the fantasy they enjoy indulging in and that's alright.
Having this permission will make things work smoother. For one, you know everyone is on board and the player knows to look for what is going on - and also wants to be part of it. For another, since you're only engaging people who are interested in this aspect you don't have to worry as much about flubbing things by not hitting the right notes. Still, there's more to consider.
Second, Consider IRL Concerns. Yes, All Of Them
Here's a thing. Games are games. IC is IC and OOC is OOC. It's asimple line to draw. It's a simple thing to state. It is not such a simple thing to practice. The human brain isn't generally good at separating fiction from reality when it comes to emotions, that is why fiction is able to tug on our heart strings so much and why fandoms go absolutely crazy at times. Because of this, you want to consider - other - IRL concerns.
Does someone who will be involved in the romance at the table have a romantic partner? If yes, is said partner ok with their Significant Other spending 2-5 hours every week 'pretending' to be in love with someone else? Especially if that someone else is a player of the appropriate sex/age who shares a hobby the other SO doesn't? Or maybe even does.
Regardless of if the SO is at the table or in the table, you want to make sure they're cool with it. Now, maybe the player will say it is none of their SO's business, but it's still generally best to ask. Games are fun, but they're not worth breaking up a real relationship over.
This is also true if the SO is at the table. Just giving them a heads up of like "Hey, my character is getting pretty sweet on your SO's character. You cool if we go forward with that?" Some will be. Some won't be. Yes, I've tended to notice being ok goes up with age - or with length of the relationship - but people of all ages get prickly about it so don't assume.
Third, Pre-Game Arranged Romances Almost Always Fail
Maybe others have better luck with this than I do, but I can't think of a single pre-existing romantic relationship between PCs that has worked out once the game has started without being incredibly forced at one point (i.e. a character's personality was almost completely re-written.) I talked about this a bit on Monday, but it bears repeating. Characters change when they go from concept existing in your head in isolation to actually in play. They change drastically. It's generally best to let romances - especially between PCs - develop naturally in play. If you do want that pre-existing relationship, maybe say the characters dated but broke it off after a bit. They can fall back into it in game, or just drift apart. It's a whole lot better than having one person sitting there feeling left out because they don't know what to do while their IC partner is off having fun with other PCs.
Four, Fade To Black. No One Needs To Squick
Sex in character is fine. Sex role played out is generally not. Just fade to black. If you need to explain how the character makes love or has sex (are they vigorous? timid? dead fish? Experimental?) but don't worry about the nasty details. Everyone's seen porn or had a relationship of some sort, they can imagine. And if they can't, no need to be having that particular talk at the table either.
Fifth, Be Careful With Your IRL Partner
This is a final note. If you are in a relationship with someone out of the game, and in a relationship with someone in the game, be careful. I've seen IC fights become real fights as players couldn't step away from their character or their view point. An "I can't believe you're looking at that elf like that" quickly turns into a real argument about expectations and roaming eyes and the next thing you know someone's crying, someone's pissed, and a door is hanging on broken hinges while a relationship lies in ruin. It sucks.
I'm not saying couples can't RP couples. They can. It can work. Just be careful. Keep your IC IC. Keep your OOC OOC. Remember that you love each other, and that just because the bard is sleeping around behind the Paladin's back doesn't mean that Sarah is sleeping around behind Lynn's, or even wants to.
Drama in the game is good. Drama at the table, in real life, not so much.