The article PhrogPhlyer posted above summarizes the issue quite while -- it's not about nudity or sex, it's about (1) profiting through use of your military status and (2) disrespecting the uniform/service.
Essentially, it's wrong for the same reasons it would be wrong for any of us getting paid to pose in uniform (or components thereof) for any media.
I get it and I agree. I just think younger generations won't agree with us.