But “obscene” has a loose definition and that is where differences of opinion can come in.
I tend to agree (talking here about the law as it should be, not as it is) that there is no rational reason for distinguishing between 2D and 3D fake images. The only rational basis for the “visually indistinguishable” criterion, in my opinion, is that there might make it too difficult to obtain a conviction over real abuse images, if the defendant could just claim that they were actually computer-generated.

The concise answer is that lolicon is legal if it is (a) not visually indistinguishable from a real minor (which means things like deep fakes and possibly very realistic 3DCG are questionable) and (b) is not obscene under state obscenity law.

1 Whoever violates, or attempts or conspires to violate, paragraph (1), (2), (3), (4), or (6) of subsection (a) shall be fined under this title and imprisoned not less than 5 years and not more than 20 years, but, if such person has a prior conviction under this chapter, section 1591, chapter 71, chapter 109A, or chapter 117, or under section 920 of title 10 (article 120 of the Uniform Code of Military Justice), or under the laws of any State relating to aggravated sexual abuse, sexual abuse, or abusive sexual conduct involving a minor or ward, or the production, possession, receipt, mailing, sale, distribution, shipment, or transportation of child pornography, or sex trafficking of children, such person shall be fined under this title and imprisoned for not less than 15 years nor more than 40 years.

Any person who, in a circumstance described in subsection (d), knowingly produces, distributes, receives, or possesses with intent to distribute, a visual depiction of any kind, including a drawing, cartoon, sculpture, or painting, that—
1
(A) depicts a minor engaging in sexually explicit conduct; and
(B) is obscene; or
2
(A) depicts an image that is, or appears to be, of a minor engaging in graphic bestiality, sadistic or masochistic abuse, or sexual intercourse, including genital-genital, oral-genital, anal-genital, or oral-anal, whether between persons of the same or opposite sex; and
(B) lacks serious literary, artistic, political, or scientific value;
or attempts or conspires to do so, shall be subject to the penalties provided in section 2252A(b)(1), including the penalties provided for cases involving a prior conviction.