I'm usually the disciplinarian in DD situations and dominant in romantic/erotic situations. When that occasionally switches (power exchanges), either because I deserve discipline or just for fun, my GF usually focuses on my penis.
When I really deserve to be disciplined, it is very humbling to have to switch (exchange power) with my GF and ask her to punish my penis (usually a pretty severe whipping, along with some additional CBT). This is not at all pleasant (very uncomfortable and/or painful), but I appreciate being held accountable and I learn my lesson! This discipline usually doesn't take very long and is not explicitly sexual. In fact, from a certain perspective, it's oten almost anti-sexual, because she often finishes my punishment by locking my severely chastised penis into a male chastity device for at least 72 hours... occassionally up to a week... on one occassion for 10 days! This is much worse than having to wear the device for 24-48 hours as a type of foreplay. If you've never been denied all erections and having your penis touched by your partner, yourself (not even in the shower or standing over the toilet), or even by fabric (your underpants, sheets, etc), for over 72 hours, you can only imagine, but not actually have any idea how unpleasant this punishment is, especially right after a severe penis whipping (no chance to alleviate any discomfort with some physical touch).
When I decide to exchange power with my GF strictly for romantic reasons, I find a good penis spanking to be the ultimate in power exchange. Occasionally giving up my dominant role, to the point of humbly having to ask her to be dominant, and then obey her instructions to strip and present my bare penis for a spanking (actually, swats with a ruler and/or a short leather strap) is a huge turn-on. These erotic encounters play out over a much longer period of time than discipline (in the form of foreplay)... teasing me with a combination of tender stroking, squeezing, pinching, and rythmic swats with the ruler/strap. None of this is uncomfortable or painful... even the swats sting only enough to drive me crazy with sexual arousal. After 30-45 minutes of this "torture" (especially if my penis has been locked in a male chastity device for the prior 24-48 hours), I am ready to explode... literally!