When a player is hogging the limelight like this, the way to deal with the situation is to stop encouraging them. The player is getting their fun by having everyone's attention focused on them (see this question for a similar situation). (This isn't a bad thing, by the way! It just means you have to make sure that the rest of the group gets their fun, too.)
Don't Reward Indecisiveness
Right now, it sounds like the entire group is (unintentionally) rewarding the player by watching when he's roleplaying, letting him stall, and otherwise paying attention to him. So the first thing you need to do is to stop paying attention to him. That sounds harsh, but doesn't have to be.
If the player refuses to make a decision in-character, the DM should simply say, "Okay. While you're thinking about that, I'm going to switch scenes to the other characters for a bit. Let me know when you've made a decision." Then the DM swaps focus to the rest of the group and ignores the player (except to answer the occasional reasonable question, such as a request to make a knowledge roll to help make a decision). This is going to be difficult for the DM, because the player will immediately do everything they can to get the attention back - asking lots of questions and demanding immediate answers, trying to RP more stalling, etc - but it's up to the DM to repeat, "I've given you all the information your character has. Let me know when you've made a decision." Then the DM must return to ignoring the player until they take action.
Similarly, the other players can help by not letting the player sidetrack or delay discussions. I've DMed and played in a number of games where one person (in- or out-of-character) couldn't make up their minds about what to do. If the rest of the group couldn't convince them to take an action in a couple of minutes of discussion, the other players would just forge on ahead, telling the character IC, "Catch up when you're ready, or stay here and guard the horses, it's up to you." Then either the player would say, "Screw it," and go along with the group, or the player came up with a reason to stay behind and sat out the following scene(s).
In cases of slow, drawn-out RP, it's perfectly reasonable for the NPCs with whom the character is speaking, to get impatient and interrupt the PC, talk over him, make assumptions, or otherwise do any of the things actual humans do when they get impatient. So if the player is giving a drawn-out William Shatner speech about "My... disease... (cough) is getting... (cough) a lot... (cough) worse... (cough) and I need...", most NPCs are likely to interrupt at this point with, "Yeah, yeah, you need the local cleric. Two streets over, third building on your right. Have a nice day!" Then the NPC will back away quickly and refuse to speak further (because who wants to spend any time around someone who's obviously sick and coughing up gross phlegm?). Note that the NPCs don't have to - and shouldn't necessarily - answer the question that the PC was actually going to ask. They're impatient and grossed out; they're going to make an assumption and skedaddle before the PC can correct them. The player will quickly figure out that if he doesn't get to the point, NPCs won't stick around long enough for him to talk to.
Do Reward Fast Movement
Rewarding the player when he acts quickly, whether by making a decision, roleplaying smartly, or otherwise playing in a more group-friendly way, will help encourage him to do those things more. For example, have NPCs be more helpful if he gets to the point quickly, or compliment him on his swift decision-making if he doesn't dawdle.
It's going to take a while - and a lot of work on the entire group's part - to encourage the player not to hold up the game for everyone. The player may respond to initial attempts to speed things along by being louder, or going even slower than usual. Ignore him, don't give him any attention, and don't let him slow down your game, and he'll eventually figure out that being a valued party member is much more rewarding than being the guy everyone is ignoring.
This sounds like player behavior
I'm usually the one to point out way to address an issue in game. Not this time. I don't think this is a play style thing. From your description it sounds like this guy is having a little too much fun getting a rise out of you.
The player's behavior is really a bit out of line. Constantly hitting on every NPC of course requires you to play their part. (And that's been more fun for him than for you.) He may be hitting on you, or just trying to provoke a response. Either way it's not OK. You've made it clear this is no fun for you, and he hasn't changed his behavior.
The Details
Hitting on the celibate priestess, well that's just a little "challenge." There's no problem with someone wanting an in-game girlfriend who's a powerful figure like a chief priestess or a powerful adversary. Abandoning his party and getting a PC killed, that's not cool, and the other party members won't be real keen on that.
But that's all in-game stuff. And what you describe goes a little beyond that.
Respect
It all comes down to respect. If a player is disrespecting you (or anyone at your table) then that is cause to eject him from your game.
Fantasy vs. Realism
Since answering, the question has been updated to specify that "realistic relationships of all kind are OK." But while you might be able to encourage a person to be more sensitive and patient in a their courting efforts, it probably isn't realistic to expect them to fantasize what you want them to.
If the player would like to role-play like a chick magnet from a Heavy Metal comic, it's not likely you'll convince him it would be more fun if in-game courtship were more like "real life."
Going forward
If you're not ready to disinvite him yet, let the table know that the game is going Rated PG. (I'm guessing most of your players will be happy with this.) Don't role-play the romance with the fellow. You could allow the dice decide whether he impresses a female, and describe her reaction matter-of-factly, all in the third person, as boring as possible.
If he doesn't simmer down, you need to ask yourself if you are comfortable with the behavior. If you aren't, then he's got to go.
Best Answer
Talk to the player
You have done this; good - do it again.
This time explain the rulesTM.
You need to decide what the rulesTM are. My suggestion is that when he:
If he is willing to live by the rulesTM fine - if not, explain that its time for him to find another group.