Responses
It's not that hard to believe when they are literally animal-human hybrids. Animal heats are pretty mindless (that's why they call rough sex animalistic) so they probably already have a more forgiving mentality around sex.
Also Kyoga doesn't remember anything so he doesn't have any trauma or bad memories around it - it makes it a lot easier to forgive since he already forgot. Also they're what? 17-18 now? that's 4-5 years in the past even more time for memories and feelings to fade. It honestly would have made less sense if he made a big deal out of it when he can't remember jack.
I still can't believe Kyoga forgave Akira that easily. That rape scene when they were 13 is so fucked up. I saw Kyoga's ass bleed. Kyoga forgave Akira too easily. I wish there was more substance to this. I loved the world building and the concept of demihumans aliens infiltrating earth, but the noncon just gets too much.