well... different guys do feel (and look) different... but given that i can have an orgasm that is stimulated by my finger i guess a guy would have to be very small indeed for me not to be capable of orgasm with them...

if looking at his penis isn't doing it for you then i guess you need to find other things that you can do with him that do do it for you. maybe... instead of there being a distinction between 'foreplay' and 'the act' there needs to be a more gradual progression of whatever it is that you enjoy doing with him.