What the article describes is not a crime.
Sexual Offences Act 2003 (England and Wales):
11. Engaging in sexual activity in the presence of a child
(1)A person aged 18 or over (A) commits an offence if—
(a)he intentionally engages in an activity,
(b)the activity is sexual,
(c)for the purpose of obtaining sexual gratification, he engages in it—
(i)when another person (B) is present or is in a place from which A can be observed, and
(ii)knowing or believing that B is aware, or intending that B should be aware, that he is engaging in it, and
(d)either—
(i)B is under 16 and A does not reasonably believe that B is 16 or over, or
(ii)B is under 13.
(2)A person guilty of an offence under this section is liable—
(a)on summary conviction, to imprisonment for a term not exceeding 6 months or a fine not exceeding the statutory maximum or both;
(b)on conviction on indictment, to imprisonment for a term not exceeding 10 years.
Adults are guilty of an offence only if they believe the child is “aware” and if the sex is had in the presence of the child (rather than without the child present) for sexual gratification. Adults arguing they believed the child was not aware do not need to show their belief was reasonable, only that it was their belief.
The article suggests that the parents believe their child sleeps through when they have sex, and also that the presence of the child is because he is in the room permanently for co-sleeping.
Co-sleeping parents who have sex are not generally breaking the law.