I'm against them because I see the bigger picture (no pun intended).
1) Lots of girls gets boob job.
2) This distorts the statistical distribution of female body shape. In other words, we see more small girls with big boobs than what natural occurs in nature.
3) Girls see this and become insecure about their body shape (which is perfectly fine, normal and -- to me -- beautiful).
4) These girls then get boob jobs.
5) This distorts the statistical distribution of female body shape even more.
6) This, in turn, causes more girls to get boob jobs or girls with boob jobs to get even bigger fake boobs.
After a few generations, all our women look weird -- i.e. we have the Michael Jackson syndrome.
Finally, since there is no such thing as a good, safe penis implant, I don't want girls to get breast implants. The last thing I want is a girl with fake boobs making fun of my tiny-boy-like penis that acts like a frighten turtle whenever it's exposed.