Romantic entanglements are just another character-based plot hook.
You can satisfy your notoriously annoyable player along with the more interested players by keeping this in mind: it doesn't have to be fluff. I've had an NPC romantic entanglement as a player (well, it wasn't all that romantic, but the setup is the same).
Some people really like tons of extra detail that doesn't necessarily touch directly on the main plot (or any plot), frequently because that makes the setting feel richer and more "real" and helps them get into the minds of their characters. Others simply like certain types of story element or subplot, and will enjoy a story that has that sort of thing in it. Still others dislike everything but the most pragmatic, efficient dissemination of story elements and guidance through the plot.
But I've never heard someone, of any player archetype, complain that a story element or subplot was woven too well into the main story or the game overall.
Adding a romantic subplot for a character definitely can feel like something that is apart from the story, and matters only to the character(s) involved as roleplaying/worldbuilding/fluff. That can be a problem from at least two angles:
- The subplot necessarily doesn't involve all players (and leaves them
with little to do but watch, unless they choose to act out)
- The major points of that subplot only matter to players that are
really invested in the relevant character's story
But, again, things don't have to be that way! Treating the romantic entanglement like any other plot hook, along with prep work to support it, is another way to integrate content some players want to see with the adventure players hope to have and the story that you want to tell.
If the love interest NPC wants to associate with the party just to play the tambourine and pull their significant other away from what the other players want, that's a serious problem. The players exist, and the NPC doesn't. If the NPC is woven integrally into the story in some way that affects all players then it's just another NPC that it's important to interact with, and there can be an extra dimension for certain characters to consider.
Any experience you have with making plot hooks engaging for players is already exactly what you need to tap here, without much modification. Major pitfalls include:
- Not giving the romantically-inclined players what they're looking for
in a romance subplot (talking to them about what they want from these
stories can help)
- Building these NPCs only for this dimension of interaction with
only the relevant PCs (above all, the NPCs should be interesting plot hooks which affect the story, not just a specific character)
- Shallowness (a poorly designed character with little backstory or few
defining characteristics who exists to "check the box" for romance
adds little to the game for anyone, unless a player specifically
wants a shallower NPC experience. This counts double if you use any "fade to black" moments)
- Failing to plan a role for the NPCs which adds challenge or dramatic
tension to the story (as above, they should be story-relevant
characters that a PC happens to be involved with, not a hanger-on a
PC drags into a story in which they are irrelevant)
- Failing to balance "screen time" (a PC that is not interested in a
romantic subplot should get other content meant for them, and not
simply get less attention because they don't want this aspect of the
game for themselves)