You have to talk to either Ashley or Liara after each main mission in the Normandy and be nice to them. If you were nice to them and everything in the conversations went smoothly you will get a cut scene just before you go Illos.
There is also a sexual encounter in Mass Effect. If you complete the mission, Citadel: Asari Consort, and you express unsatisfaction after the mission, you'll be rewarded to a short sex scene.
Kaidan and Liara are the love interests for the female Shepards.