The ones I have looked at allow a sort of erection - just one that is a little, ahem, squeezed. If you want a major, throbbing, erect penis that is useable for sex then the above problems apply. However, if you want to feel the sensation of having an erection, albeit a smaller one, then most of the devices will allow that. The tissue can still swell, just not to the full extent, and it may be uncomfortable. ANd bear in mind, it is easier to maintain chastity if you are not erect all the time - often the purpose of the belt is to make erection uncomfortable in order to initiate negative reinforcement so that erection is only possible when commanded and allowed.