In earnest 'sisters are doing it for themselves' DIY spirit (not an innuendo. Was it? Annie seemed pretty serious about the jobs and stuff. Dave was not smirking), my contribution to the lexicon of 'doing it':
Do It as straightforward euphemism for sex - Not used by anyone other than 10-11yos, '80s comedians e.g. Victoria Wood, The Young Ones. So puerile. Mumsnetters are far more sophisticated so say 'DTD'.
XXXs do it in/with YYY - puerile but universal 80s sex joke trope, popularised on bumper stickers (whatever happened to those?) and instantly, involuntarily recognisable to anyone who remembers Yuppies, Rubix cubes and Adrian Mole.
Birds do it, bees do it, even sentimental fleas do it - deffo sex, though the song says 'fall in love'.
'Just Do It' Nike slogan - about sport but streams directly into the inner 'sex' part of people's brains, as so many advertising messages are designed to do, thus inducing a far bigger response than any sporting slogan should. The 'death row last words' derivation may be true but is esoteric knowledge, it's not what the phrase is about in Nike's useage.
Do-It.org volunteering - Follows from the Nike useage, so not about sex but may access unintended brain areas.
Do It Yourself aka DIY - home improvements, popularised in the 70s. Used smuttily as an innuendo for masturbation.
Relax, don't do it, when you want to go through it.... when you want to cum - not sex. No. Specifically orgasm.
do this / do this thing - not about sex
Anyone else?