But for a person with a sexual preference for teens, their attraction is

  (a) something they can't control

  and

  (b) something that doesn't harm anyone if there is consent between both parties.

Honestly, I haven't seen much evidence for point (b). It ultimately comes down to experience and power-dynamics. We can accept that two people of the same age do not have a drastic advantage over each other. It is much less likely that a thirty year old does not have an advantage over a fifteen year old. Even if that advantage is one that is not acknowledged by either party, it exists to some degree.

A minor is someone who we have arbitrarily decided has less ability to determine how to act compared to an adult. Yes, different cultures having different ages indicate that this is arbitrary, and yes it is difficult to know where to draw the line exactly, but the fact is that to protect minors the line needs to be drawn somewhere. It is mildly contradictory that a 16 year old in Britain can sign up for the armed forces, have a child, and get married, but they are not allowed to drink or drive a car. That doesn't mean that those boundaries should not exist, however.

Ultimately the exact definition of these sorts of things come down to cultural handshakes. Why do people drive on the left-side or right-side of the road? It's a cultural handshake. Consent between two parties of different emotional maturity is a difficult thing to figure out, hence why we have the cultural handshake that says that for certain parties it is unacceptable for them to liasion at certain ages.

To say that those power dynamics apply to two adult men of the same age, is -- to put it mildly -- ridiculous.