Well, there is charge conservation as well as lepton number conservation. So at the vertex a (electron) neutrino can couple to a W+ and a e-, since the lepton n

Well, there is charge conservation as well as lepton number conservation. So at the vertex a (electron) neutrino can couple to a W+ and a e-, since the lepton number of the neutrino is 1 and the reaction products do not have a total charge. ( But energy-impulse conservation dictates that at least one of the two needs to be virtual.) Actually there is something called effective field theories, which is essentially a technique to derive simple rules to calculate specific processes. ( For example, one can derive a effective theory containing only up and down quarks from the standard model. Such a theory would be sufficient for nuclear physics, since only up and down quarks are present in protons and neutrons.) In such a theory, which only describes the interaction between neutrinos and photons, the neutrino would acquire a small but non zero charge. ( Or equivalently the photon would be charged under the weak force.)