I agree it's YTA for calling it a lie but I do think that its ok he put a stop to sexytimes till he gets used to it. Prosthetics used in sexual body parts are kinda in a weird place in terns of disclosure. Prosthetics of all types are body parts and need to be recognized as such. I know breasts are more than just sex organs but they are used for sexual acts that both parties can derive pleasure from. Her breasts are a bit different and that changes the sexual activity a little. What if he enjoys breasts because touching them gives his partner joy? Now he doesn't know if she can get physical pleasure from being touched there or not. He was anticipating her body to be one way and got thrown for a loop understandably. Maybe not have the conversation in that moment but if he needs time to adjust it's better they do other things instead then discuss how it might impact their sex life with a bit more maturity.