Otokonoko Succubus-san to Amaama Seikatsu

The series of otoko no ko (girlish boy) succubus continues!
As each ejaculation made is a waste of magic power... his partner human recommends that he tries out dry orgasming, which isn't accompanied by the ejaculation of semen.
This succubus boy's prostate is going to be trained.

(Source: DLSite)