oral sex can be classified a sexual act in which one's mouth is used on another's genitals....usually a stimulant [foreplay] sometime a afterplay....
and whether oral sex is legal under sg law..
http://www.lawgazette.com.sg/2004-9/Sep04-feature2.htm
Fellatio/oral sex
There appears to be a linguistic struggle to include fellatio within the scope of s 377. The law was clarified in two cases. In the Singapore High Court case of PP v Tan Kuan Meng,41 Justice Lai, after citing Indian cases,42 held that fellatio ‘between a man and a woman as a lustful substitute for and not a prelude to and enhancement for natural sex between them is carnal intercourse against the order of nature’. The Singapore Court of Appeal affirmed this decision in PP v Kwan Kwong Weng,43 holding that ‘when couples engaged in consensual sexual intercourse willingly indulge in fellatio and cunnilingus as a stimulant [foreplay]44 to their respective sexual urges, neither act can be considered to be against the order of nature. In every other instance the act ... will be ... punishable’.
basically in layman term... it says...
if No sexual intercourse after oral sex.. then it is against the law..can be classified under unnatural sex......
if got sexual intercourse after oral/fellatio sex then it might not be against the law.
for more info goto the link i provided above..