Hentai: Been trying to find this for years.
A girl getting fucked in public (Unsure if it was by multiple guys/monsters) while being watched by some form of a leader that's getting his dick sucked by a girl that the other girl is affiliated with. While getting his dick sucked the leader isnt hard until the other girl starts to get fucked as he gets hard from her being violated.. It takes place in a fantasy type setting.
i thinks its this one if pic related:
http://hentaigasm.com/kuroinu-kedakaki-seijo-wa-hakudaku-ni-somaru-1-subbed/