I’m looking for a series of doujinshi that’s about this guy and his friend meeting meeting their two crushes and the main character gets invited, by his crush to a hotel. At the end of the first one he’s locked in chastity. The rest of the series is him getting slowly transformed
No idea if it’s the one you are talking about as it’s not exactly as you describe but it sounds a bit like “Futanari! Punishment time” by Piririnegi
If you find the name of the series please tell me also it sounds like a cool series