My husband and I always pet both my M2 and U2 under their wings. I know they say you're not supposed to, but it's their favorite thing also. I never have a problem with sexual stimulation. Tiki is almost 5 (harmonal teenager age) and he would just die if I didn't pet him there. The U2 is only 16 mos. old. Only two times while petting him (not necessarily under the wings),
he did seem to get a little stimulated, but I just told him to step up and got his attention on something else. I was a bit surprised, he is so young. Anyway, I think it's ok as long as the bird is just enjoying it and not getting stimulated. I have read where some people only have to be careful with this during breeding season.