Assuming the romance between two characters is appropriate (i.e., within acceptable cultural guidelines), in terms of their ages, there’s another factor that can sometimes give readers pause: the balance of power between them. I’ve recently come across this in a novel that takes place in feudal Japan, and I’ve been running up against it, as I write Shinosuke.
Basically, if one character is in a position of relative power over the other, it makes readers a little squeamish, when a romantic relationship starts up between those characters. Teacher-Student is one example, even if the student is over eighteen, and another is Employer-Employee. There is also the potential for this issue to come up any time the age difference between the two characters is more than a few years, especially if one is between, say, eighteen and twenty-five. The reader finds herself wondering if the romance is real, or if the subordinate character simply feels that they have no choice but to go along with what the dominant character wants. Or, in the case of a large age difference, the younger character may be subconsciously dominated by the older character. (This, to me, seems less of a concern in a contemporary story, than in a historical. Young people growing up in the modern Western world are no longer raised to automatically defer to their elders. Kids today!)
This is particularly an issue in feudal japan, when dealing with the relationship between a samurai and a commoner. Samurai had the right to kill any commoner who displeased them! So, getting back to my story, from the get-go, the relationship between Shinosuke and Senpachi is imbalanced. Obviously, Senpachi has no intention of harming Shinosuke, but Shinosuke has no way of knowing that. Even if the samurai says, “I would never harm you,” Shinosuke would have to be pretty naive to believe him. And even if the character believes Senpachi, the reader might think Shinosuke is being a stupid teenager.
(The issue I had with the first third of the samurai novel I’m currently reading, which is otherwise well written and enjoyable, is that the lord made it clear that the other character’s life was at his disposal several times, imprisoning him and punishing him in ways that would have Amnesty International sending reports to the U.N. While it was perfectly in keeping with the culture and period, I had a hard time sympathizing with the main character falling in love with this man. On the other hand, I suspect it was meant to appeal to the BDSM crowd — of which, I am not a part. *)
So, what to do, what to do?
Well, step one is to make damned sure Senpachi doesn’t ever mistreat Shinosuke. He’s teaching the young man bushido — the Way of the Warrior. And as a teacher, he will have to be stern. But most readers have seen enough movies like The Karate Kid or even Kung Fu Panda (which we watched last night — not bad!) to recognize the difference between stern and sadistic. Whether I can pull it off will simply depend upon my writing ability.
Step two is to make the romance almost entirely Shinosuke’s idea. There is simply no way to have Senpachi broach the subject without it appearing that he’s abusing his position as mentor to the young man. In fact, he will have to put up some resistance. His attempts to rebuff Shinosuke, and Shinosuke’s hurt over having his advances refused, will, one hopes, eventually make the reader sympathetic to Shinosuke’s cause — i.e., winning over the heart of Senpachi. We then move from “Why is that lecherous older man hitting on his student?” to “Why can’t that jerk see how much his rejection is hurting Shinosuke?”
Welcome to Romance Plotting 101!
*NOTE: It’s also a common element in manga, which may be more pertinent. I’m a fan of manga and anime, but I often find the dominant/submissive elements of the stories not to my tastes.
Jamie Fessenden's Blog 
Great analysis – it is certainly a “modern” problem, I think, given the various civil rights movements of the last 100 years. Which, IMHO, viva la progress! 😉 But you are right, it is discomfiting for most modern readers (discounting those for whom it is a kink/fetish, of course). I do *like* it as a plot device, if the author has thought about like you clearly have: as a serious issue to be addressed at some level.
My question is born of cultural ignorance here: *would* Senpachi view sexual relations with his student as a breach of trust? Is that part of (for lack of a better phrase) the samurai code, by which I mean, very strict sexual morality?
Also, would there be an element of class discomfort there, at all? One thing I’ve always had trouble buying, despite it being a classic trope, is the governess/lord (or maid/lord, or servant/master) scenario and not just the power imbalance, but the cultural one — Jane Eyre only works as a romance because Rochester doesn’t care what polite society thinks of him, or her. Would a samurai like Senpachi have those concerns at all?
That’s just me being nosy, not questioning your decisions; what I know of samurai history and culture could fit in a thimble. I just think it is interesting to watch you address this problem. Voyeur? Moi? 😉
Those are all very good questions. At the time, it was quite common for samurai to take on young male lovers — very young, as in between the ages of 10 and 15. The relationship was often one of mentor/pupil, so Senpachi wouldn’t find the idea itself repugnant. But of course, a modern reader would. So there’s a balance to be struck there.
I’m having Senpachi’s friend, Kurobachi, tease Senpachi about his obvious attraction to his student, and pushing him in that direction, but Senpachi is resisting, due to the fact that he believes it is duty to train the boy to be a warrior — not to be his bedmate.
I’m currently reading a book on homosexuality in feudal Japan which EM Lynley recommended. I’m not sure yet what it has to say about commoners being the lovers of samurai, but it may not have been unheard of. Certainly, it was a common practice in China. As far as Senpachi goes, he has been banished from the court of lord Mashikura, so he may be disgusted with courtly life, at the moment. But he is samurai and proud of it. His main motivation for teaching the boy, though Shinosuke is really too old, is that he feels the boy was shortchanged by his father’s death and deserves to have the training that should have been his due, had he been raised as samurai.