There are already good answers here, and my own is not intended as an alternative so much as a supplement. Talking to the problem player is absolutely the right avenue to pursue. What follows are some suggestions about how that conversation can be structured to help align everyone's goals.
As I understand your situation, you have several distinct problems which happen to intersect often:
This player apparently has very specific things they want to explore in your game, and those things fundamentally do not involve the other players in any way
This player is turning away from the plots you've prepared, and replacing them with activities for their own character alone (again, excluding the other players)
This player is taking spotlight time and dedicating it to narrative descriptions which are unrelated to the plot
This player's expectations of freedom to pursue whatever they want to do have been reinforced by your decisions as DM
Your other players are becoming frustrated as the game is filled with content that neither interests nor involves them, and actively impedes their ability to pursue things they do want
(1), (2), and (4) can probably be grouped together, as can (3) and (5).
What does this player want? [Items (1), (2), and (4)]
It's important to go beyond the surface of "the player wants X, because they always do X" because often behaviors at the table are specific attempts to get the things they want, and those attempts in themselves are not necessarily what the player is looking for.
You can talk to the player about this:
I've noticed that your character concept features sexuality pretty prominently. What does that mean for you, as a player, at the table? What kinds of events do you imagine happening around your character, and what kinds of stories are you interested in playing through to explore those ideas? What sorts of things could happen in the game that would bring these themes out for you? Are there situations or consequences you can think of offhand that would seem unfair to you, or would take away from playing this character for you?
The answer may not be satisfying-- if the player says "I want to read my hardcore erotic fiction at the table", then the problem remains. But if the player is interested in romantic subplots with sexual mores of your setting as complications, that's something that you can work with.
Political intrigue and sexual intrigue often mix pretty smoothly (for a loosely medieval reference, see Game of Thrones). The character's liaisons with unimportant NPCs are... unimportant, but toss a plot-relevant noble into the mix and you've aligned the main plot with this character's personal plot, along with some story hooks and an incriminating secret for the NPC and PC both.
If the main story doesn't include the elements this player is most interested in, they may be making use of the freeform nature of TTRPGs to seek out those elements on their own. If you've allowed this so far, without consequences, then the player may well have concluded that this is the way to get the content they want. You can address this too with a conversation:
Most sessions it seems like you take your character away from the story I've set up to do other things, and those things seem kind of random to me and have been hard to fold into the story. Are there things that are missing from the main story for you which I could add to make it more interesting? If not, would you be willing to follow the story prompts more often (not all the time, necessarily, but more often) so that the story can play out?
Letting the player know that you don't like behavior X at the table is OK, but it's much better to explain what behaviors you do want, and why. That changes the discussion from "you're bad and ruining the game" to "I want to run the most fun game possible, and I think it would help me do that if you could do more of Y".
It's also the case that you've got a huge amount of scope to shape these activities, whether you're prepared for them in advance or not. The player does not have the authority to declare that they seduced some NPC without consequence. If the player wants to seduce someone, you can make it a skill check (so failure and unintended consequences are possible). It's also within your authority as DM to explain that, while this PC may be pansexual, a given NPC is not, and will not respond well to the PC's advances. And you can impose any consequences you feel are appropriate-- think about the experiences a real-life person might have if they constantly try to have sex with everyone around them, all the time.
Finally (for this grouping of items), you are running the game, not this player. If a player wants to deviate from your planned content, well, that happens. Often. But content you've been able to give some forethought to will frequently be better than content you try to devise on the fly while following a player's lead. Having a clearer idea of what this player wants at the table will only improve your ability to provide content that interests them. But it's worth remembering, explicitly, that you are in charge of the game world and the game.
This player has no scope to dictate the circumstances of the game in the situations you describe any more than they can simply declare that an enemy has a heart attack and dies, or that they randomly found a +100 sword, or that they are suddenly immortal.
The game is for everyone, and being unbalanced in favor of some players over others is a distinct problem, regardless of other factors [Items (3) and (5)]
The most important thing to keep in mind with this set of items is that it doesn't matter what, specifically, this player is doing or why. The core problem is identical whether a PC is pursuing explicit sexual encounters as it would be if the PC were demanding to play out an audit of the royal accounts: no player should have the game focused on them at the expense of others all the time. Remember that the problem here isn't the specific content this player is inserting, but the impact the player is having on forcing the story off the rails and curtailing other PCs' options.
A player taking the spotlight is always a problem-- everyone should have their moments to shine, if they want them. No amount of character backstory justifies such an imposition on the other players. And this player is not entitled to you as a DM, your game as a backdrop, and the other players as their audience. If they really want to run through this character's sexual odyssey, they can do that just as well writing at home, alone. If they want to play D&D, they're more than welcome at your table, but the game is not just about them and their character.
In the situation you describe, there are some additional factors, namely that the player is describing events which have already happened to them alone (among the PCs), and so there is no scope at all for the other players to participate at all. Some of that is OK, but it doesn't take a lot to be too much. As above, speaking with this player is the best way to approach what is a meta-game issue:
I know that you're excited about your character and have put a lot of effort into the backstory and personality you want at the table. The way things have been going your character is getting more attention and influence at the table than the others while also not advancing any of the story. The game is for everyone, and so the other players need to get their share of time and attention at the table, too. And the story should continue so that the players that are interested in it can enjoy it.
Solutions to this can be tricky, since making a metagame argument can feel very arbitrary. But if you can get a better handle on what this player wants (as in the section above), and are able to incorporate that into your game, you may be able to get this player to follow your lead more instead of haring off on their own.
And again, it's not about the content this player is offering (even if it does make players at the table uncomfortable), it's about moving the game forward so that every player gets to participate in things that interest them. Hearing monologues from this player is not that. Keeping the focus on the game will do a lot to de-personalize the problems you want to discuss.