can't request a chat, wait until you feel comfortable until the topic develops in a natural progression? like if sex scene comes on TV. You may begin with something like, "Speaking of sex, can I ask you something?".
-catch your parents in the right mood. For example, choose a time when everything is getting along and there are no issues to blur the discussion.
The right mood does wonders so be attentive and roll them into your love story with careful introduction :) Good luck!