Erections only happen because blood pumps to the penis. Therefore, necrophilia with a guy is impossible.
To reiterate what BlastingNaba said, Angel lust, or "death erection" is a thing. In addition, according to that wiki page, a "death erection" likely occurs when the death is "swift and violent", like for instance if someone slams you on the head with a brick.
WHONKSex.Koishi-chan pressing a little boy for physical relations.