Seido Maria ~Hakudaku Mamire no Zangeshitsu~

Main character lost his parents in an accident at an early age and was brought up at a church. He grows fond of facility manager girl, but his feelings only get ridiculed. One night he takes a walk and sees lights in one of the buildings. He peeks inside and sees his love interest having an affair with multiple men right inside the basilica...

[From vndbreview]