Deepthroat - Describing oral sex wherein one individual's penis ends up deep in the throat of another individual.
Dildo - A prosthetic typically designed to resemble a penis for the purposes of penatrative sex or masturbation.
D.I.L.F. - Dad I'd Like (to) Fuck
Discreet - Term used to describe an individual that is gay/transgender/queer/etc but does not openly identify as such and prefers to engage in queer sexual activity without allowing individuals other than those involved know about said activities and/or the relationship(s) and/or identities between those involved.
D.L. - "Down Low". Essentially another term for discreet.
Double Penetration - There are several similar sexual acts this term can be used to describe involving either three people, two people and a dildo, or one person with two dildos. One variation of this act involves one individual being penetrated by two other individuals' penises within the anus or vagina at the same time. Another variation of this act involves one individual being penetrated by two other individuals' penises at the same time - with one in the anus and the other in the vagina.
D.P. - Double Penetration
D.T.F. - Down To Fuck