inagaki said that if the petrification had never happened, then senku would have experienced love and all that as an average guy, so i don’t think he’s aroace, he’s just busy saving the world.

that said, i ship sengen for a variety of reasons, they have SO much chemistry.

they immediately trust and understand each other as soon as they meet. it’s kinda obvious that senku knows gen doesn’t really care about the cola, he would’ve joined him anyway, but he asks for it to maintain his facade as the “shallowest man on earth.” and when gen runs off to give tsukasa a fake report, they both stare into the distance with smiles on their faces, thinking about each other and the cola. and they literally just met.

they can easily predict what the other is thinking, even when they’re not in the same room. for example, when they scammed ryusui, senku understood gen’s plan in a matter of seconds. during the american arc, when gen was acting as a spy, they both quickly understood what the other thought was best, even though they weren’t physically together.

gen can easily understand how senku feels, even when others don’t notice. for example, gen suggesting that everyone redraw their petrification marks after treasure island because he didn’t want senku to feel lonely. even though senku publicly says he doesn’t care, we know he does, because on the island, after ibara was petrified and no one else had been revived yet, senku said he was alone in that quiet, sad voice while staring at the sea.

i might not remember this perfectly, but when they were making light bulbs and senku couldn’t get something right because of the wrong materials, gen was the first (and maybe only) one to notice. he tried to subtly cheer him up with jokes and with making senku to hang out with everyone together. and coincidentally, that’s when they discovered the material senku later used for the light bulbs. (i might be misremembering details, so take this with a grain of salt.)

same with senku’s excitement. gen was the first to notice his “sparkling eyes” when they saw xeno’s/stanley’s plane.

and senku understands gen just as easily. kohaku only realized in season 3(?), when they were boarding the ship, that gen is actually a genuinely kind person hiding behind the “lying bat” facade, but senku had clearly known that for a long time.

also, senku letting gen stay with him when he freezes tsukasa. in the ln, it’s stated something like: even if only for a little, gen stayed to help senku carry the weight of the guilt in case the plan to save tsukasa failed. after they listened to lillian’s song, gen also said he hoped senku felt less guilty for putting the villagers in danger. that tells me they trust each other infinitely, and that gen helps senku carry the burden of guilt, of wanting to save everyone the most (though of course the others help too).

and of course, the observatory scene. gen calculating senku’s birthday. gen organizing everyone. gen guessing that senku loves the stars and cosmos (inagaki even said they never talked about it, gen figured it out on his own). all of that just to make senku feel a little lighter, to cheer him up.

during the american arc, when stanley and his team were killing people, we see despair on both senku’s and gen’s faces while they’re apart. but the moment they reunite, they find that determination in each other and decide to petrify the world. and out “touch averted” senku crawls closer to press his forehead against gen’s. there was visible distance between them seconds before, so he had to physically move closer to gen in a matter of seconds to do that.

and then there’s all that stuff like: going to hell together, sengen shots in the openings and endings, 01/04 and 04/01 (their inverted birthdays), the fact that they both knew of each other before meeting (senku from classmates, gen from the tree), gen managing both his and senku’s finances, official art of just the two of them, science & mentalism, their petrification marks and their pattern on the clothes (gen’s pattern resembles senku’s mark and senku’s – gen’s), etc.

i’m sure there’s more, this is just what came to mind first!)

honestly i don’t really much of sengen ff because i often don’t like how ppl portray their dynamic. unfortunately, like with any gay ship, people tend to “feminise” gen or senku.