An additional clause will make it illegal to pay for sex with any 16 or 17 year old. It is not otherwise illegal to pay for legal sexual activity.
Should paying for sex with a 16 or 17 year old be illegal?
Other (see comment)
Update: Sandy - any 16 or 17 year old, ie paying for sex with someone who's 16 or 17 will make otherwise legal sex illegal. (It will also be an offence to pay for illegal sex, eg U-16, or with someone U-18 who you have a duty of care for, but that's already illegal..)