Inspired by Kei/@sky_skyhime headcanons,
Senku falling first for Gen when Gen had gifted him the observatory. He was pondering as to why his cheeks felt warm and his heart rate increasing. Was it heart palpitation? What about cheeks feeling hot? Maybe that's just because of the cold.
He started to pay closer attention to Gen, noticing he started to feel that way only for Gen ever since the observatory gift.
It took him only after a night in the observatory where Gen was fast asleep and he was watching the stars. He looked at Gen, he tilted his head, looking at Gen. He sometimes would wonder that Gen's eyes look similar to the starry night. The way it sparkles when he is planning some mischievous schemes or pretending to be innocent. It's all such a charm, and his brain finally clicked with his heart.
All those warm cheeks and heart races were because he loved Gen. But knowing about it doesn't help. They are in the middle of stone wars with Tsukasa. He doesn't even know if Gen likes him that way. He doesn't want to be irrational and the rational thing is put his feelings on the backseat and move forward to help the kingdom of science and winning the war.
Gen fell in love when Senku cheated and won against him the poker games. Gen always had adoration for Senku. Since the tree carving and gifting him the observatory by expressing his honest feelings and actual name to Senku after a while. He always thought his feelings were just admiration and reverence for the man, being the mad scientist and a warm human being. But by the moment he realized his love for Senku. It was too late. They are going to America. Who knows what trouble may arise considering Senku's track of bad luck.
That's why, he kept those newly realized feelings to himself. He didn't want to burden Senku with his feelings and he wasn't in the mood to get rejected in the middle of this mess.
When the shooting in south America started. But they both thought even if they get saved by the petrification. This might be their final chance. When they crawled to each other. Forehead to forehead. They both confessed their feelings, before they started to laugh at how long it took them. They shared a soft clumsy kiss. Gen's heart was beating fast due to all the deaths he saw but now it feels less heavier; he and Senku happily fell asleep to the beam and constantly thinking about their "final" moment in their head. Keeping Gen conscious with a happy memory instead of the deaths he saw.