A person aged 18 or over (A) commits an offence if—
he intentionally engages in an activity,
the activity is sexual,
for the purpose of obtaining sexual gratification, he engages in it—
when another person (B) is present or is in a place from which A can be observed, and
knowing or believing that B is aware, or intending that B should be aware, that he is engaging in it, and
either—
B is under 16 and A does not reasonably believe that B is 16 or over, or
B is under 13.
A person guilty of an offence under this section is liable—
on summary conviction, to imprisonment for a term not exceeding 6 months or a fine not exceeding the statutory maximum or both;
on conviction on indictment, to imprisonment for a term not exceeding 10 years.