Four out of five (children) with an ASD are boys.

1) Boys like to masturbate.
2) Boys with an ASD often do not like wearing clothes.
3) Boys with an ASD may have limited understanding of public protocols.
Boys with an ASD who like to masturbate may also not like to wear clothes and may also have a limited understanding of public protocols.
Boys with an ASD may find it easy and comforting to expose part or all of their anatomy for the purposes of masturbation.
Boys with an ASD are known for the above, in public places. (I will provide references, if requested.)

1) Girls (probably) like to masturbate.
2) Girls with an ASD often do not like wearing clothes.
3) Girls with an ASD may have limited understanding of public protocols.
Girls with an ASD who like to masturbate may also not like to wear clothes and may also have a limited understanding of public protocols.
Girls with an ASD may find it comforting to expose part or all of their anatomy for the purposes of masturbation; however, it is less likely to be easy. Although casual clothing for girls is likely to be similar to that of boys, it often still has peculiarities with regard to fasteners, layers, and textures.

In addition, some cultures are likely to be more tolerant of a boy touching himself (clothed or not) than a girl.

So, whether or not a girl does actually masturbate in public may not be a matter of 'want' but of opportunity. Boys (seem to/are recorded as) avail themselves of such opportunities more frequently (obviously?) than do girls.

I say again in response to Louise18's question, "Can anyone think of any autistic traits that affect one sex much more than the other?"

Public masturbation.