What I always got out of the Thai food remark is just that Thai food smells salty; when people have sex, they sweat. So I guess her crude remark was just to state that the air smelled salty with sweat, which is what Thai food smells like.
"F*cking", which could be spelt like Foking, which is kind of Thai-sounding when you say it. xD So when she said "It smells like Thai food - have you guys been f*cking?", the f*cking part is tying in to the Thai part.