If a guy is nervous or slightly uncomfortable he may not cum. It's the old fight of flight instinct kicking in. As soon as anxiety occurs, even at a subconscious level, you can kiss goodbye to a happy ending! If a guy suffers from performance anxiety he can usually get it up but may either lose his erection during sex or never reach orgasm. It's all about feeling totally at ease with the situation and enjoying it and if necessary releasing repressions related to sexual pleasure. It's certainly not something that should be taken personally!
Alternatively if he's an AAS user and his E2 is elevated following a cycle or he's on Deca he will have a hard time finishing off and may not be able to get hard at all!