This was one of the things idea guys convinced Chris to do, most people didn't see it because Chris didn't post it online, he shared it with Joshua Wise who shared it with a user here.
He told Chris that the only way to give birth to Crystal (Chris-chan's long awaited dream daughter) was to have sex with Sonichu. Idea Guy told Chris all he had to do was draw the said act.
This, in my opinion, was the only thing idea guy did that I found genuinely funny. I'm sure it brought Chris some anguish because he thinks this is all real, yata yata-
But I just think how Chris reacted to this is funny, since he was told this was he only way to have Crystal Chris responded with "

I guess I have no choice but to anally rape my son..."
I mean, he could have just not done it, it wasn't like idea guy was threatening him with moon lasers at this point, but it would just mean no Crystal for Chris and he just couldn't accept that.
I also find it funny that Chris didn't have Sonichu saying something comforting like "okay father, if it's the only way- I understand!" no instead he drew Sonichu screaming in pain and agony.
and that shirt too... wearing a shirt saying "Love is Love" while doing such a heinous act... I'm sure that Sonichu will never look at Chris the same way again.