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Balance It Out: The DM can set the tone from the beginning by balancing a bit of seriousness with fun. It's okay to laugh at sex, but for every scene in which romance is used for comic relief, try to add a scene in which romance matters. Maybe the whole group gets a good laugh when the hapless barbarian discovers his prostitute is actually a wererat - but only after he's naked and separated from his silver weapons. Maybe further down the road, the group is concerned when the paladin's girlfriend shows up with amnesia and no memory of who he is. By mixing it up, you let the players know that they can't expect every lustful encounter to be the same. Players can also try to vary their experience by having their characters seek a fun date one time, and an intimate date later on.
Small Moves: To acclimate players to a game with romantic elements, start small. Let one major NPC have a sensual motive that the party is likely to take seriously. Maybe the party is sent to kidnap a nobleman that an evil cleric deeply desires (or, on the flip side, they're sent to retrieve him from the cleric's clutches). Either way, the rewards and adventure will happen, but this introduces themes of sexual conquest and possession. It won't seem so strange later on, when the party comes face to face with an alchemical aphrodisiac that really works, but also disables spellcasting ability for hours after the fact. Players should also start with less complicated overtures if romance is a new in the game or new to them. A player character could have some quick flirtations along the road, building up to looking for a partner when they're back in town.
Use NPCs First: Introducing romantic elements via lesser known or distant NPCs can be the best way to start, since you have more leeway with NPCs in any given situation. Players can take experiments with their characters very personally, and they can feel proprietary interest in their character's friends, family, and related NPCs. If it seems like you are picking on NPCs they like, the players can start to feel resentful and to treat all NPCs like stage dressing. But people they don't know well have their own lives and are ripe for drama. Over time, you can move closer to the PCs, until you involve them directly in romantic plots – or they try to create hijinks of their own.
Start with the Shiny: There are many ways to involve the player characters in attraction and courtship, which are easy places to start. Player characters tend to be shiny, with higher ability scores, fine clothes, enchanted gear, and plenty of extra loot. They also tend to have the worldly experience, endurance, and mystery that come with being adventurers. Unless they're slumming it, player characters are going to stand out and people are going to come calling with everything from flirting to gifts to offers of marriage. There is plenty of ground that can be covered before ever getting to an outright sexual scene. Players should also keep their characters' best qualities in mind when they enter the scene. You might actually try to get extra attention from the opposite sex by donning your character's best clothes for a night on the town.
Assert Yourself: This is the same place that players can start if they're interested in romantic storylines. A player can ask the Dungeon Master to include a lover or spouse as a part of their backstory, so the character starts the game with an attached NPC. Or a player can find an NPC during the game and make friendly overtures. It might still be best to start with NPCs so that other players don't feel uncomfortable, especially if you don't know the other players very well. You can then try to shape the encounter by the way you address the DM. If you approach an NPC in character, speaking from your character's viewpoint, then you are trying to create a scene. If you ask to make dice rolls and keep things out of character, then you are giving the DM clues that you want to remain removed.
Play to the Room: Sometimes people feel uncomfortable dealing with romantic exchanges, especially when the DM and the players are of the same gender. To cut down on awkwardness, it can help to adjust your style. Some people feel silly trying to sound like the opposite gender, but if your group is having fun, chances are they won't mind some experimenting. If you start to feel better about it, you will have a fun skill under your belt. If you don't, however, there's no rule saying that you have to do extensive voice acting. Just describe what people say so everyone can hear it.
Using the third person when describing romantic details can create a sense of distance and literariness. "When the village invites the party to lunch with them, your character notices that the same girl is smiling in his direction, but trying not to look too long." Including dice rolls (and perhaps focusing on them) can help to reinforce the sense that the scene is a part of a game, with strategy and goals. "He catches up with her and tries to make friendly? Which type of roll is he going to try to make to do that?" Limiting the time that is spent on romantic scenes can put the group at ease, as well.
Fade to Black: Roleplaying the graphic details of sexual encounters might not be the best choice for a few reasons. Unless every romantic encounter involves the whole party, it's likely that most of the group will be off to the side, bored or wondering why one player should take up so much time. The Dungeon Master should keep an eye on the clock and get back to the rest of the party with short delays, to keep everyone entertained. And unless your group is interested in a minute-by-minute description, you will want to guide scenes so that they end in a "fade-to-black." You've seen the technique in films, when lovers close in as the camera steadily goes dark. The scene ends and the audience knows that sex has occurred off-screen. The next scene starts later on, and the story continues. The sex isn't ignored, but it isn't shown.
It can end up sounding something like this: “She grabs you by the hand and carefully sneaks upstairs while the party continues. Roll for avoid being seen. Although you almost stumble, it doesn't seem like anyone notices you. Roll to see if you can find an empty room by listening at the doors while she plays lookout. The two of you make it to an empty room and lock the door. She says, 'I wonder how long we can stay up here before someone comes knocking...' Are you willing to find out? All right, then. You spend the rest of the party with her, and sneak out the window when the family is heading to bed.”
Blue Book It: Some people suggest that, if players want to continue a sensual scene with great detail, the DM should arrange to do so later on, in writing. When we were in high school, my first DM would pass folders with lined paper to his players during the day. He would start a scene, then they would write a reply and give the folder back to him so he could continue it. This back-and-forth exchange could go on all day. Since it's now 2010 (and not the mid 1990s), it would be easy to ask players to carry on in email, on a message board, or some other electronic medium. The DM could deal with the results later, and other players could choose to read the scene if they're interested.
Don't Bait and Switch: It's important to remember that many people come to the table expecting a particular genre, and return to the table because they want to play that genre. Gamers can also enjoy a steady supply of variety: some combat here, some roleplaying there, with hijinks and mystery thrown in. If sex takes over and the rest of the world falls by the wayside, some players might feel like they've been cheated out of the game they signed up for. Although you should run games you enjoy, you must always keep the players in mind. If you discover that you want to run or play a more sex-based game (or dispense with the game and stick with sex fantasies), you might want to start another game for that purpose. Don't change the major tone of a game without being sure the group wants to follow.