In short:
Let him do what he wants to do, but guide him and help him with your knowledge. Make sure he understands what the chosen profession/class means and what things it offers
Long Answer:
First off, those kinds of learning experiences are actually good for him. He has a certain thing in mind that he would like to play, so why hinder him? in DSA (german abbreviation for The Dark Eye) there are many professions to choose from, and thus many different characters to create. Let him gain experience and see what he has in mind.
Now, how can you help? Have you told him what you think? Talk to him about it, ask him if he really wants to be a wilderness focused character and if he knows what that will mean in the game. Help him design the character in a way that he is a good fighter and hunter. That is (very much so) possible in DSA. It is actually one of the great strengths the system has.
You could also show him the alternatives directly. Show him what a dwarf warrior from Xorlosch (which I assume is the dragon hunter warrior you mean) gets as skills and talents and what the big game hunter offers. Also keep in mind that with a profession that does not cost as much GP/AP (depending on what version of DSA you're playing, I can't say anything about the creation prices in Version 5, as I only play 4.1) will actually allow him to still spend points on combat skills and be a good fighter.
If you're concerned that he's avoiding classes he would like more because, as you say, he's been mocked online about only playing warriors, you could also try and tell him how different a warrior in DSA is to other games, and how it is essentially an important and interesting character, not only someone who takes the front row in a fight and does nothing else.
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
It's everyone's job
Long story short, being entertained at a table is a shared responsibility. GM should welcome all players and create opportunities for the entire group and it's also up to the players to take advantage of these opportunities. Making an interesting character helps. Having your players create characters that fit the story helps even more. Doing the character creation together is strongly encouraged in many games. Not just the characters contribute to an interesting game. In play the GM should give player's many opportunities to do something engaging, develop their characters and have fun. It's up to the players to take those opportunities and use them further.
While you are not required to have a backstory, it is very useful to the GM. I've often used the rule of 6: every player needs at least 6 sentences of backstory, 6 paragraphs are recommended, but 6 pages is too long. I would recommend that you have some backstory that the GM can use, it's good for your own roleplaying as well. GM should accommodate for everyone present at the table, though, regardless of their backstory status. However, in your case the backstory problem is secondary to other underlying issues.
If one player gets much more attention thanhis peers, the group has a problem
It is true that some characters are easier to "hook" than others, but little backstory should never be used as an excuse to ignore one player. The fault is never equally on one side of the table, but what you said points to a case of GM favouring one player above others and taking over the play. As you said, that player made his character with the GM (while others I assume did not have that opportunity), is being given extra time and attention and gets to do much more than the rest has opportunity to.
This question has some excellent answers on how to deal with such a situation.
I suggest you leave the group at this point.
There is little that can be done to fix it if your GM is unwilling to listen and cooperate. To make the case clear:
A GM that rages because you have a shitty character is a shitty GM. A good GM would help you create a better character if that is indeed the case.
A GM that railroads and prevents players from branching out does not understand player agency. GMing is not playing with dolls. A good GM recognises that the most important feature for a player is freedom of choice.
A GM that is deaf to his player's feedback is doomed to fail. A good GM understands the symbiosis between himself and the players and listens to players carefully.
Actually, any person that asks for critique and then rages is hardly worth having around. It's fishing for compliments, not actual discussion.
You GM is not beyond redemption, but he will have to rethink the way he runs the game very carefully.
You may want to try and help him become a decent GM. I personally wouldn't. If you still would like to play with this group, you can offer to GM yourself or if the GM wants to listen point him to some good GMing for Beginners tips. You can also point him to this page.
Regarding round length
Note that sometimes a group has a Party Face, a character that acts as the main speaker and communicates party's intention to the NPCs. Many groups take turns doing the talking, but some prefer to have one character be the go-to talker. If you're uncomfortable with it, you need to communicate to the other players. In either case, turns should be shared, meaning that anyone can speak if they are a part of the discussion.
However, in combat, the turns should be as long as appropriate - there is no prescribed time that you need to fit to your turn. It's sometimes necessary for some turns to be longer (a cleric performing a complex multi-attack routine while activating magic items and casting quickened spells) and some shorter (I hit him with my sword!). However, in the long run everyone should get the same amount of **spotlight* and feel contributing and appreciated. That means that while the cleric takes more time describing what he does, the GM would devote more attention to the fighter, describing the effects of his actions in detail.