He's an expert martial artist, pretty godly infact, he could just pull out right before he cums. There's also anal, bj, etc...
Edit: Not to diss on the author, but it's pretty obvious the anticipation for the intercourse is one of his hooks to keep the readers. I just hope he doesn't take it too far like "Kekkon Yubiwa Monogatari" where the author blueballs the MC and the readers to keep them reading and throws ridiculous reasons like "chivalry" or whatnot when the girls themselves are literally spreading their legs not knowing when they will die in that apocalyptic world. It's only making love, doesn't need to be explicit, but since lots of ecchi readers anticipates it, lots of authors abuse this anticipation just to keep their readers around.