But prostitution doesn't do this. A commodity has full or near full fungibility, i.e. "one sex" is (almost) substitutable by any other "one sex". Obviously this isn't true for sex.
For this commoditization to be meaningfully damaging to women (which is what the GP claims), it must cover both prostitution and relational/emotional sex. The kind of sex you have with a prostitute (legal or not) is substantially different from the kind of sex you have with a lover.
But prostitution doesn't do this. A commodity has full or near full fungibility, i.e. "one sex" is (almost) substitutable by any other "one sex". Obviously this isn't true for sex.