Saitou is older and more experienced in many areas than Sano. He’s married to a woman, with whom he may or may not have children, and also lived with a bunch of active and presumably attractive men in close quarters during the war. He has demonstrated more effective combat techniques than Sano, which may easily translate into more effective physical techniques of another type. It is perfectly natural to assume, therefore, that Saitou is more sexually accomplished than Sano is, and in fanfiction we authors often assume exactly that.
HOWEVER. Though suggestive if you want it to be, none of that actually definitively indicates that Saitou is more sexually accomplished than Sano. Though a wife is mentioned, the Saitou of Rurouni Kenshin is not stated to have children, and of course there’s very little indication of sexual activity on anyone’s part in canon. Beyond that, Saitou is kinda single-minded about justice and his duty, to the point where, when we write romances for him, we often have to drag him, annoyed and frustrated at the distraction, away from justice and duty. For all we know, Saitou could be very inexperienced in sexual matters, possibly even a virgin.
Meanwhile Sano, who not infrequently gets labeled with the big V because he’s fairly young and has been concentrating on stupid fighting shit for all of his adult life, could actually have been having tons and tons of sex with anyone and everyone ever since puberty. Nobody’s going to argue his level of sexual attractiveness, I think, nor that the settings in which he most likely grew up, in between Sekihoutai and Kenshingumi, would be propitious for a lot of casual sex. And sex could well be another way, in addition to fighting, for him to forget his past. (Actually that’s basically how I picture ASZz Zanza.)
So what happens when virginal or near-virginal Saitou meets sexcellent Sano and the inevitable eventually occurs? I imagine it would drive Saitou up the wall.
I’m assuming a sexual Saitou here, obviously (and a sexual Sano, even more obviously XD). Of course we usually do in fanfiction, but with this idea it’s important to make the distinction. And for a sexual person, the first instance of really good sex (sometimes the first instance of sex, period) often makes a huge impact.
And I can just picture poor Saitou obsessing, entirely against his will, over this mind-blowing experience he’s had, completely unable to stop thinking about how much he enjoyed it, how different it was from anything he’s ever done before, and about Sano himself.
I’m thinking this arose from a one-night stand (though how Sano managed to get that to happen I’m not sure XD), with neither of them considering anything more than that. But of course they have subconscious romantic feelings for each other (because that’s what happens in
my life fic about Saitou and Sano), and at this point Saitou would be forced to examine those feelings. Because what, he would wonder, does it say about Sano, whose talents he has always denigrated, that he is so incredibly good at something that has made such a huge difference to Saitou and that Saitou can’t stop thinking about?
It’s just adorable to think about Saitou hormoning like a teenager and incredibly irritated about it. He’d be lusting after Sano and super annoyed at him at the same time, and I think we all know how I feel about Saitou sending mixed messages to Sano. And then what if Sano became aware of this obsession of Saitou’s? He would be such a little shit about it! And if Saitou was going crazy before that point…
I don’t know if they’d be fuck-buddies for a while before realizing their true feelings, while Saitou wondered if something was wrong with him because he was so irresistibly attracted to this idiot and fixated on sex (and eventually learned that nothing was); or if Saitou would try to hold out against Sano’s seductive powers and become increasingly irritable and hormonal as he saw Sano around and attempted to keep his hands off… maybe he would try and repeatedly fail not to have sex with Sano, which would, of course, make him feel bad about himself and his own will power, and he’d have to come to the realization eventually that getting into a relationship is OK even when you’re a badass government spy with very important Justice to dispense.
In any case, I think Saitou would have to wrestle with questions about whether it’s appropriate to be so overcome with physical desire; whether that desire is the same as, wrapped up with, or distinct from love; and what it says about him that he’s so very susceptible to these physical passions. It would indeed, in many ways, be very much like what a lot of teenagers go through at puberty, and (I know, this is so much like the sad assumptions idea) Saitou would probably feel distressingly young and lost with the experience.
And Sano! I know I said he would be really obnoxious about it, but only for a while, I think. He would certainly revel in having any kind of power over Saitou, but I think when he realized that in this part of life Saitou was, for once, not as advanced as he was, he would be a little more kind and helpful. (Especially since even an overwhelmingly aroused and obsessing Saitou wouldn’t put up with his shit.) And he would probably be really turned on by Saitou’s uncontrollable passion for him as well as by having Saitou looking up to him in any area.
Saitou is regularly so effective, typically has his shit so much together, that I find I’m really charmed and intrigued by ideas wherein some vulnerability or weakness he secretly has is revealed, especially if Sano then has to help him with whatever it is. I also like to see Sano as an expert at something, since I feel like he has the capacity for mastery but is very much in a learning and growing stage of his life in canon (and therefore doesn’t demonstrate much expertise in anything except hard-headedness). And I think those two states, in these two characters, would come together very well for some interesting interaction and relationship-building.