Behaviour such as kissing in public is currently unpunishable under Singapore law. The only related ordinances present in Singapore, such as section 27A of the Miscellaneous Offences (Public Order and Nuisance Act), includes a prohibition on nudity in public places, or appearing in the nude in a private place that affords a view to the public eye. Such offences are punishable with fines not exceeding $2,000, or imprisonment not exceeding 3 months. In addition, the carrying out of indecent behaviour such as obscene or sexual acts is also prohibited in public places under section 20 of the same Act. If found guilty of the offence, the parties involved are liable for a fine not exceeding $1,000, or imprisonment not exceeding 1 month.
It is probably easier to just ask the couple to get a room.